miss in exhaust at idle and acceleration

I have a 96 chevy blazer with 4.3 vortec.Engine has been rebuilt due to low compression on 2 and 6 cylinders.Found that head gasket was blown during tear down process.Heads were checked lapped and sealed in rebuilt process.Engine did have this type of miss and engine shaking before rebuilding also.Thought after rebuild miss would surely go away,but has not.I have also changed the fuel regulator before.Approx. 5-6 months ago because truck would start when cold and had trouble starting after it was warmed up.Replaced regulator and fixed problem.I am not a mechanic but I do know basic maintance.