Cadillac Forum - Enthusiast forums for Cadillac Owners

hello all and thanks for any help in advance.

Heres my problem, I bought a 1997 seville with 130k miles in good shape, took it home and it overheated, took it back to the dealer and he replaced the thermostat, radiator, and water pump.....
Picked it up and drove it home and once again it overheated, had the dealer come and get it for repair again....he took it in and had it for a week and said he resurfaced the head, changed the head gasket, timeseted it.

I picked it up again and it doesnt overheat anylonger, took it for a 200 mile drive to test it..

2 days later it started idleing rough when started and the rpm would jump from about 600 too 800.....then go away for awhile as it was driven, until i took a exit ramp and it backfired as i was climbing the ramp.

Slight misses from 60-70 mph and in idle.

I bought this car3 weeks ago and the dealer hasdone alot aleady, but I owe him 3k yet for the purchase of it and hes taking payments as he is a friend of mine.

what would cause the problems, as before all the work it did not sputter at all.

here are the codes i pulled:

pcm p0603
p1578
msm b 2120
b 1656

thanks in advance