Hello,
I've found the strangest problem with my transmission. At first it'll shift normally, then after what seems like a random amount of time driving at around 45mph, it shifts to a lower gear and won't shift back up to 3rd. It will start doing this anywhere from 25 min down to 10 min of driving. If I come to a stop for a bit I can sometimes get it to shift back up into 3rd, but it'll drop back down soon after. The vacuum line is hooked up and the kickdown isn't pulled until open throttle. It was working normally up until recently. Any ideas on what could cause this? Thanks in advance.

 

1st thing comes to mind is a failing vacuum modulator. That and a fluid/filter change will be the easiest and cheapest thing to fix.

Blow out the metal transmission modulator line and vacuum source port too, to make sure there's no carbon blocking it and causing vacuum issues. Are you getting any white smoke out the tailpipe? which is a sign of a ruptured VM diaphram.

Past that, I think you have a THM gremlin that may turn out expensive.

 

If you look at the powerflow/application chart. It looks like you are losing pressure to the direct clutch. You could try a fluid service but im my experience it does little to nothing. If the fluid is black and dirty the damage gas already been done and changing the fluid is a waste of time and money. If the trans is a high mileage unit it might be hardened internal seals.

 

Check your kick down cable to make sure that the inner cable is moving smoothly inside the outer sheathing
also check to make sure that your Detent mechanism is working.
I had a similar problem and found that my new replacement Kick down cable kept getting hung up inside the outer sheathing.
the cable would engage the Detent on WOT but would not let it release. The trans would not shift back out of its down shift position.
ended up replacing the cable with one from The Parts Place. problem gone....
Good Luck

 

I’d look at the governor. Make sure the bore it rides in isn’t chewed up, make sure the nylon gear is solid on the governor housing, abs make sure the end of the gear isn’t chewed up.

 

Update
The kickdown cable was getting stuck in the sleeve, I pushed it in and heard the mechanism close and disconnected it. I drove it and the problem came back but I discovered if I shift it into neutral then back into drive it goes right back to the proper gear for a while before dropping down again.

 

I forgot to post a follow-up to this and a related thread. I found the culprit and ended up rebuilding the transmission.