94 K1500 5.7 timing question.

[Smallz72project]

I recently swapped motors in my 94 with one from a 92 the guy just rebuilt. I installed a brand new distributor, got it running but noticed I had it one tooth off so I fixed that and had a buddy help me set timing. Ever since then, if I drive at low RPM it chucks like it wants to die or has a really lumpy cam. And if I am just slowing down at lower rpms it does it as well. Starts up, and drives at speed just fine on the highway. Another thing I have noticed is my mpg suck!! I expected it to with this truck but Im only getting 9-10...I figure I should be getting at least 13-14. I have suspected timing to be the culprit...does this sound familiar or like I am the right thought process to anyone? Thx in advance and keep on truckin