Hey guys looking for a little help,

On the 1993, V6 3.1L, Cavalier just had to replace the headgaskets, one of them had a crack and wasn't completely sealed.  Tha started about 3 year ago when my Uncle started to disemble it, he got the intake manifold off and i did the rest.  Got it all put back together and it started up.  But i noticed when idling it would run rough and stalled the first fews stops.  Once the accelorater was pressed it was fine till it got up into overdrive and it would run rough again. And i can't figure out wahts wrong? Any help greatlyy Appreciated