Technical transmission slips at idle

I rebuilt the transmission in my 1953 ford pickup. It has a ford 302 with a 1969 c4 ******. I put a Transgo shift improver kit in it. The modulator valve came with the adjustment screw almost all the way unscrewed. I turned it in some, but not sure how to adjust? When the truck sits at a stop light, you feel the ****** slip out of gear. Step on the gas and it locks back up. It is a non lockup converter. What do I look for to fix this?

 

Not the modulator.

worn pump, hard seals in the forward clutch... but that's just a guess

 

shift linkage adjusted on the "edge"? when motor relaxes it pulls it into neutral?

 

Adjusting the C4 modulator:-

• clockwise firms and delays the upshift; and
  
• Counterclockwise speeds up and softens the upshifts.

Make your adjustments one turn at a time, then take a spin to evaluate the upshifts. Do not adjust the vacuum modulator any more than four turns in either direction.
As Squirrel and tb33anda4rd alluded to, possibly other issues that need to be diagnosed.