I have noticed a surging/losing power problem?

1997 Hyundai Elantra mileage: 80,000. For the past two weeks since having my car serviced (grease and oil change) I have noticed a surging/losing power problem. Initially the problem was when I was just beginning to accelerate from stop, then the pattern changed to running ok when I first drove the car say 5km to the supermarket and then when I came to drive home it would surge/lose power/buck after a period of the car stopped for 20 minutes. After noticing this pattern, I repeated it and the fault happened again. The car is an automatic and I have had an automatic transmission service, had the coil replaced and the air flow meter replaced as this was the only code the testing equipment showed. After the new air flow meter, the pattern changed again to playing up intermittently and today the mechanic experienced the problem driving with the testing equipment attached and it showed no codes. I normally drive it in overdrive and economy and the mechanic thought it might be cycling in and out of overdrive but it still plays up with the overdrive off. The mechanics are stumped and are contemplating a problem in the onboard computer. I have noticed it is using more petrol than before. Up until this problem, I have never had anything go wrong with the car and it has not lost a drop of oil or water in the six years I have had it. It has had all its book services. Prior to the service, the car somehow picked up a bit of plastic on the road and it is melted to the outside of the exhaust. This is why I took it for a service in the first place. Now I am wishing I never took it to the garage at all, as the car ran fine before it and played up after but the garage assumes no responsibility for causing it and says it is just coincidence. I have also had the catalytic converter checked and that is clear. Do you have any other ideas as to the cause of the problem? I have already spent over $1200 testing mechanics theories and don't want to spend anymore dead money. I would appreciate any help you can give me.