TH350 shifts fine into third when on power, but not when accelerating slowly?

I bought a truck this weekend in SC and drove it back to VA just fine with no issues, no shifting issues at all. Today driving to work i noticed that just rolling nice and slow, lightly on the gas the tranny was surging between 2 and 3. I step on the gas and it goes into 3rd. Now the transmission has a shift kit in it, when you stand on the power maker it lights the tires up in both 2nd and 3rd. Can someone please advise if this is normal due to the fact that it has a shift kit in it? Any help is appreciated.

I've not experienced that with any of my th350 s which I've had shift kits installed.

Check adjustment on detent cable

I checked my kickdown cable this weekend to make sure it wasnt adjusted to tight (which it wasnt), and then screwed the modulator all the way out... Still searching between 2-3 when not on the gas... ?

Are you sure it's the tranny and not an engine surge? A mixture problem at mid-range cruise might cause surge as it's being held waiting to shift? Just a thought. How much vacuum are you pulling, and are you sure the line from the manifold to the modulator is like new and not leaking? Assuming the line is good and that you have good vacuum at the cruise range where you're seeing the problem I'd buy a new modulator and eliminate that from the list of potential problems. Sounds funny that you backed out the adjustment screw with no effect.

Since I'm not comfortable messing with automatics much I'd probably stop by our local independent tranny guy's shop, talk it over with him and see if he'd take it for a quick test drive for me. But then we have a fantastic old school tranny guy that's honest as the day is long, and I trust him 100%. I'd avoid the "big name" national chain shops like the plague, unless you know for sure that the guy running the place is more than a shyster fleecing the unknowing.

I have not done a vac test, so I cannot say for sure. I did notice a difference when I am on the gas, it shifts later, but still no difference when just lightly accelerating or at a stable speed. I will check the vac, and replace the line ends that are rubber, and then if that makes no difference I will replace the modulator. Thank you for the help.

I think it could be one of, or a combination of, the adjustments to either/both the vacuum modulator or governor. I recall after we rebuilt my TH350 we played with both of those and got it to shift just right. This link touches on it if you click on those two items but nothing in detail about how: TH350 Transmission - Tuning - Tech - Hot Rod Magazine

Have you tried changing the tranny fluid and installing a new filter?

No I have not but I will give it a shot, 36 sedan.

Without knowing exactly whos shift kit and the part number, it's a guess. But the govenor would be my first suspect. Since it's in a truck you may be able to remove it and see if it's moving freely and not sticky. You could even replace it with another and test the operation with a known good unit.

The modulator can over-ride the govenor to control down shifts based on engine loading (vacuum), but the govenor is the primary shift controller based on driveshaft speed.