Intermittent power issues

New here. My mrs has a 2009 Jimny, early this year it randomly cut out whilst leaving a round about. Shortly after I tried it out and it started and worked fine one day then the next it randomly lost power doing short trips. We took it to 2 garages who did the usual checks and found nothing. 1 garage had it running for 1 hour before it cut out and for 4 hours before cutting out. They suggested changing the crank shaft sensor. This didn’t change anything. When the engine cut out it was a total loss of power and the car wouldn’t connect to any diagnostic tool, and when it did it gave no faults. It would cut out at different times under different loads. When I drove it, it cut out leaving a roundabout again and there was a total loss of power, all dials and electrics went.
The garage suggested maybe an ECU fault and gave me the car back. Tried driving home an only made it 15 minutes before it cut out. It now hasn’t started since, tried a few different times and still won’t start. Decided to get a second hand ECU kit and so far I’ve swapped the ECU only and it didn’t start. I’m planning on replacing the ignition key, controller assy? and something else that came with the kit I bought. 
Looking for any advice on this please. I’ve got basic experience and the garage said everything they checked was okay, although that was a few months ago now and it’s been sat on the drive since. Has anyone had intermittent power issues? Is it an ECU issue? I’ve found someone that can look at the ecu and look for issues but I can’t seem to kind any that work or have databases on a jimny ECU. I’m also now not sure the battery is sufficient. I was thinking of rechecking the starter and alternator but I have no experience in that, happy to learn, as the engine ticks over and sounds okay just no ignition but I need to double check.
Thanks for any advice. Hope I’m using this forum right. 

Hi. ECU change will need the key to match it. If losing power, and dials going off etc, have you got a bad connection somewhere? Will take some chasing, but possibly a bad earth.

If there is a TOTAL loss of power, no lights, etc., that's your starting point, the electrical power from the battery, through the main fuse panel, ignition switch, etc.

You're losing connection somewhere.

First off, thank you for the replies and advice. I've just gotten round to swapping the ignition key and this time to car started and ran for 5 minutes! I started and stopped it about 4 times and it sounded fine and reved up fine. 
The radio showed a system check so I thought I'd use my dads Foxwell car scanner and it wouldn't read the cars VIN (it wouldn't read the VIN before swapping the ECU) so I manually entered the VIN and then ran a quick scan. After the scan (Im not technical in this and I can't remember what exact scan I ran apart from selecting quick scan) it finished the scan but said it no communication and it failed to scan. It has done this before, months ago when at the garage that tried to fix it. They said they had issues connected and only managed to scan it a couple times bringing up no faults, or couldn't connect at all. So then I tried to restart it and it wouldn't start again! I thought maybe the battery is low and put it on charge.
I then decided to change 2 other control modules, I'm sorry but I dont know what they are exactly other than one says Controller Assy 4WD and the other has a blue label. I plugged the new ones in, so I have no replaced the ECU, ignition key and 2 control modules with second hand ones and after charging the battery for a couple hours it still wouldn't restart. 
Now I'm confused as to why it started and seemed fine for that short period, I looked at the grounding points in the engine bay and they look fine. I have bought a little multimeter so I can start to trace and try test some cables and see if I can learn to chase and test some power and earth points. Oh I also checked the fuses and none have blown! Any tips on how/best way to get into testing the cables I'll be grateful, thanks for the advice so far. Enjoying the learning even though it is testing my patience and frustrating!

You need to be VERY specific as to what is being done and how you describe what the results are, we are not there, we cannot see.

When you say it wouldn't start, do you mean it doesn't crank or it cranks and doesn't start?

You're previous statements say "total loss of power" which to me means, nothing works, swapping the ignition key (I'm assuming what was swapped was actually the ignition switch), which you're saying you've now done, isn't going to fix that because many of the circuits are not powered from that circuit.

Sorry for the mistake and incorrect terminology, I'm new to this.
I mean currently it cranks but doesn't start or should I say ignite? There's electrical power and everything lights up, but I feel as though the battery is draining or getting low. And yes I mean the ignition switch.
Before when I said total loss of power I meant whilst driving the car cut out as if it had stalled but lost all power, lights, dials, etc. This happened a couple of times seemingly randomly. After the power returned but the car would only crank and not start. Is this better? I have changed the ECU, ignition switch, control assy and a control module which sits next to the control assy? I was going to change them back to original and do some voltage and continuity tests.