Ok, so I have a 91 Z28 with the automatic trans (700R4) and recently it developed an issue where the OD or overdrive gear doesn't run smooth at all and D or drive is sometimes sputtering between 30-50 mph.

Ok so lets say I just pull out of my drive way when in OD I get to about 40 or 45ish (speed limit) at a normal gentle cruise pace it will start to sputter and have issues getting to the next gear. But if I put my foot down a little harder it will shift down and be smooth until I have to brake and it gets around 30 or 40-45ish. I could put it down to D and it will smooth out and only sometimes give me an issue. I use it maybe 30 miles a week so its not like I have any long trips. Any idea what this could be? I checked the RPMs and the gauge needle isn't bouncing it seems relaxed through the sputtering.

List of recent work in case (last month) all of this was done by my dad's long time friend who owns a decent size auto garage business:

- new distributor, wires ,and plugs
- corrected misfire
-3" cat Borla cat back

Really worried that the trans might be going. The car has 141,xxx miles and never rebuilt and never sat. Friday the 13th it is scheduled for inspection so I wanted to have an idea what the issue is and if I can't fix it myself. I've been told by them that inspection on Friday the 13th is pretty ballsy.