2001 rodeo please help me

[jdog826]

this is my first post and i don't know to much about cars so please bear with me.
here is the deal. i got a 2001 rodeo sport 3.2l 6 cylinder automatic 95500 miles. it was given to me by a friend. when he gave it to me he told me it burns oil and has some stalling issues. it had these codes 0436, 0507, 0300, 1295, 1515, 1125. i took it to the mechanic, they put in new battery, spark plugs, fuel filter, pvc valve, serpentine belt. i've driven the car about 300 miles and the codes have not returned, no CEL. here is the problem. i start the car let it warm up drive away no problems. now if i let the car cool for 15-20 min and try to drive it, the car starts acting funny. when i give the car some gas or try to drive at a constant speed the rpms are up and down like its losing power. and when i come to a stop the rpms are up and down and sometimes stalls. any help would be greatly appreciated thanks.

[FL 3.2L]

Intake manifold gasket leak. Look at the FAQs or search for it. Very common in these engines.