overheating, check engine light, low coolant light, coolant in radiator on 1998 Chevrolet Lumina

New head gaskets, new radiator...still found coolant in radiator. New oil, filter. Pressure check ok. Check engine light keeps comming on. New gas cap and replaced light sensor twice. Mechanic said drive it around for a week and see if the overheating and low coolant light comes back on and check coolant reservoir daily.
NOW WHAT? Any ideas appreciated.

2 answers
have them check the intake gasket the 3.1 liter is known for them
Turns out I had a pinhole leak in one of the heads. Replaced it and every thing is now OK. Was told this is not surprising because the coolant required by Chevy is very corrosive on the aluminum used in the Chevy heads.