Thanks a lot for all your replies. My favorite answer is definitely the one from George_San_Jose not only because he gives me at least some hope that there is a chance to fix the car without going bankrupt but also because I think I can rule out most other possibilities:
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the fuel filter has been replaced twice over the last 3 years (once on purpose and once “by accident”) plus the last time I filled up before the engine started misfiring was at brand name gas station, so most likely it wasn’t the fuel then.
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the car has only 130,000 miles, I am the 2nd owner and the 1st owner really took care of this car, so even considering that it’s a Chevy I don’t think the engine should be seriously worn out already. Well, but I don’t know what the compression (psi) should be . . .
-Anyway the car ran flawlessly until the misfiring occurred the first time, except for the fact that you had to crank the engine 2-3 to start when cold. So I am thinking that’s maybe another hint for an electrical problem - as George_San_Hose suggested.
BTW all of you were right, high octane gas and injector cleaners did not help at all. So I will try the distributor cap + rotor next.
Thanks,
Chris