Before I drop the pan and inspect the valve body and downshift solenoid, I am looking for additional guidance with an issue that started without warning in a turbo 400 transmission. The transmission was rebuilt about 5000 miles ago and has a shift kit that shifts firm indicating good pump pressure. The issue is when in 3rd gear it acts like it goes into neutral on its own. When this occurs I can come to stop and accelerate normally and it shifts fine through all 3 gears again or manually pull the shifter into 2nd gear and accelerate normally it shifts fine back into 3rd gear. What has me puzzled is I can disconnect the 12VDC wire that supplies voltage to the downshift solenoid and still get an occasional downshift from 3rd to 2nd with moderate acceleration. I have replaced the vacuum modulator hoping it was a freak vacuum issue, however it still does a random downshift and acts like it goes into neutral on its own. What should I be looking for other than a bad downshift solenoid or sticky 2-3 shift valve? Any input is appreciated!

Follow up with a resolution to the above problem. The power brake booster shared a manifold vacuum terminal with the transmission modulator valve. The brake booster diaphragm has begun to leak allowing fluctuating vacuum to the transmission modulator. Swapped the transmission vacuum line to a separate carburetor base vacuum port and all is well now. Just an FYI to any other do it yourself'rs that stumble across this same issue...

Appreciate the follow up info, 433RED.

My pleasure Footbrake Jim, glad knowing somebody may benefit in the future