I installed a used th400 apx 3 years ago. About 1 year ago,sometimes, it seemed to stop up or down shifting when in drive. I can manually go from 1st to 2nd and drive. But when taking off sometimes it seems stuck in drive( This is how I drive it now).
Also, sometimes it seems to go to neutral when in drive. To fix it ,I would go to neutral then back to drive( while moving ) and it is fine again.
It seems to do it more when it is hot.
Should I rebuild or do I have a easier problem to fix or check?

It a 67 conv with a gm502 w/411 rears

Before you go for a rebuild:
1. Disconect the electric kick down wire from the side of the trans and test drive it. If that doesn't help...
2. Drop the pan, drop the kick down solenoid and make sure it's clean and working.

If the solenoid is open the trans goes into kick down and won't upshift or it will free wheel when you let off the gas. I once had a 400 rebuilt because an intermitant short was sending juice to the trans solenoid and caused odd symptoms like the ones you have.

Today, I dis-connected the kick down at the tranny, and my shifting is back. I think the adjustment cable was to tight. I think I'll leave it disconnected.
However, WHEN it shifts is still a problem.I test drove it 4 times today. Sometimes it shifts 1-2-3 at 2000- 2500rpm sometimes at 3000-4000 rpm. When under hard accelation it chips the tires when shifting, but then when letting up it seems the car won't go anywhere until around 3000 rpm, go past that and shifts hard into 3rd. Still if I go to neutral and back to 3rd it seems all good.

Maybe a shift kit problem at this point? maybe stall converter? It's a b&m, and wasn't suppose to be used with a big block, which is what I have.