GS300 Check Engine Light

I just bought a 1998 GS300 from a Lexus dealer with 62,000 miles. But the "check engine light" keeps coming on. The dealer has been VERY responsive in trying to fix it. It's replaced all of the O2 sensors, air flow meters, found that it had incorrect spark plugs and replaced those, etc., etc.

For the first several go-arounds, the car would chug and stall when starting or when travelling at about 40 would accerate slowly and then surge the gas pedel was pushed. That seems to be fixed now.

But the light is on again.

Any ideas?

Kenn