Jimny is sick again

So two years ago I had a few issues with it not gaining power on the highway (flat road, only going at 90km/h so would have been easy to gain speed) I would put foot in the pedal but nothing happens but then the acceleration would come back on after a minute or two (no engine light came on. Mechanic told me to drive it "hard" (doing 5000 rpm on a lower gear) for a few days and it did resolve itself. About 6 months ago my Jimny would again lose power, this time was much worse, couldn't manage 40 on a slight uphill, and super jerky, so off to another mechanic I went, was told it wasn't the cat converter, not the sensors, but the head gasket leaked and drown the spark plugs. Coils, gasket and spark plugs were replaced and problem went away. I took my Jimny for a long ride, after 40 min of driving it started to lose power (was uphill) and became very jerky, it was a busy highway so I stopped and had it towed to a mechanic nearby, he called me today and said he couldn't find anything wrong with it, apart from the fact that oil seemed a little low. I am now scratching my head don't what to do. 

Should I just replace both crank shaft and cam sensors? 
Will a mechanic tell  easily if the cat converter has collapsed? 

By the way, I notice the car was blowing out smoke when starting getting a bit more of late, not sure if it is related or just the car being old. My Jimny is a 2012 model with about 250000 km on it.

Any suggestions?

When my cat no1 lost its integrity and disintegrated I could hear the rattle coming from inside it. Listen with a stethoscope or thru a wooden stick.

But still it can be bad and not rattle. could be possible to see the underside of the cat thru the O2 sensor hole with an endoscope, but you cannot be 100% sure with taking it out for a proper inspection.
Total failure is when powder from no1 blocks no 2 cat and then it is possible to see the underside of no 1 disintegrated with an endoscope.

If driven for a while with broken head gasket the Cat can get blocked from too much smoky soot too..
But the 5000 rpm trick that one mechanic told you to do, I think that is mostly for diesel particle filters. Not so much to gain on a petrol car, but I could be wrong.

To be honest if you suspect the CAT (or want to eliminate it) it's not that difficult to remove the CAT pipe. Undo manifold bolts (2off) and then mid clamp (2off), O2 sensor and remove pipe, leaving rest of exhaust in place. You can then shine a light in at one end and see if the CAT lattice is blocked or not. Not that expensive to replace and after market CAT pipes work fine.

Personally I would be getting the work on the head checked again - 250k km is getting up there on these little engines and if the head gasket went I wouldn't be surprised if there was also damage to the head itself - did the first mechanic get it skimmed and the block checked? Get the compression checked. As Scimike said, the Cats are easy enough to check...

Thanks all for the comments. I am getting it back from the mechanic (not much was done to it). I don't have ways to jack the car up so I will go with changing the cam and crank shaft sensors first. See if it changes. On my way back from the mechanic the weekend (it broke down 80km from my house) I will bring a spray bottle to see if the same issue happens and will do a spray test.... Then let's go from there

Rear silencers collapse internally as well 
this can cause lack of power 
I've had this happen  

Last week I took car back from the first mechanic and drove 80 km to get home, little Jimny ran fine till I was back in town, 1 km from my house and right outside my usual. It limped into the drive way. For the next few days, the mechanic drove it home everyday (couldn't find any fault when started up cold, so he asked me for permission to drive it home) no issues. We are starting to have a heat wave in Eastern Australia and yesterday it was starting to get toasty so finally it played up. He said one cylinder consistently misfire when hot.....so it is a good start to work out what's wrong. If it is just one cylinder, could it still be the sensors?

Finally mechanic changed the sensor and it's good again.... Had they listened to me at the first place........