Need help! Problem with Big GS. Mexico

[Veter]

Hello, everyone! So to cut the story short I've been cruising around Mexico on my 1200gs and decided to have a service check at Mexico City dealer's. Looks like it was a serious mistake as now I have problems with my bike that I can't figure out myself. I'm near Cancun now and Cancun's dealer is said to be even worse, so I can rely on your advice only to fix it myself with those crippled hands of mine

The symptoms are the following:

1. When cold engine stalls and I need to rev up the gas in order to keep the RPMs
2. When on the first gear stalls frequently, overall RPMs are not stable and it gets worse
3. I can feel that output/power is much worse on both ends and it gets worse
4. Sound has changed - before I had exaust "caugh" only when I close gas, now it is all the time like series of caughs
5. Clutch became more tight - it is hard to hold it on first gear
6. When I try to move slower than the first gear allows using clutch there is SERIOUS vibration and "helicopterish" sound, engine stalls sometimes as well

My noob Hypothesis:

1. Smth with fuel/air mixture - need to check air filter, try without power commander (booster plug in my case), check injector (dunno how)
2. Exaust/CAT? I've been using mexican 92 fuel and 89 occasionaly. However, many ppl use the same bikes here without such problems and I didn't use unleaded fuel (which I'm not certain as they can add anything in fuel)
3. Clutch. But it doesn't hold for all the symptoms. However, I had my clutch steamed in the mountains once and asked those BMW guys to check it, they said it was OK

Please advise, I'm desperate