C4 shifting problem

[AJONE]

I have a 1967 C4 sitting in front of a 9'' with 3.50 gears and behind a 350 HP 302 crate motor. I rebuilt the trans 2 years ago with a napa rebuild kit and a Transgo shift kit and stock converter. It would shift firmly and felt nice and tight. over the last few months the trans will occasionally drag out shifts making it feel stock. it also feels like it has the slightest surging somtimes under a good load; i.e. merging on the freeway. Any ideas what the problem/solution is? sticky/plugged valve body? band adjustments? rebuild the whole thing again? I will try to reply in a timely manner to any questions.

Thanks
AJ