My 727 suddenly started getting stuck in 3rd a few drives ago right after a WOT stompy romp. It gets out of it into 1st or 2nd fine manually.

I first suspected the governor being stuck since it was not my throttle pressure lever being stuck at the WOT position (I use a cable, I have a return spring down on the transmission lever side that was still working fine)

I tried doing the trick of whacking it into Park for a split second while rolling to rattle the governor and that seemed to wake it up a little. I also poured a little Trans X in in case something got gummed up.

What I've got now is it will not shift out of 1st when it's cold. If I let off the gas it slumps back down to idle. Then after a couple more hundred feet it'll act like I have the throttle/trans adjustment set too tight and I will still need to manually select 1st from a stop. Then as it warms up further it will get into 1st on its own, but still shifts a little higher than it used to. When it is stuck in 1st it is still very peppy, no signs of trouble in the transmission aside from the shifting behavior.

I have changed the transmission filter within the last few thousand miles and the fluid is red, but I do see some gray come up on the dipstick too. No funny smells. Basically no bad behavior aside from acting like I have the throttle pressure whacked full open until it is almost warmed through.

Any point in dropping my pan and changing my filter again, or do I likely have something sticking in the governor? It is the original 4500ish rpm governor to the transmission, 125k miles.

Sounds similar to a broken sprag unit.

You can diagnose or guess.

To diagnose, get a 100 # gauge and attach to the governor port.

At start up standing still, you should have 0 #’s of pressure to max of about 1. Start to roll away from a stop and the pressure should mimic vehicle speed.

Come to a stop and the pressure should fall to 0-1 #.

If higher you have a stuck/sticky governor.
If 0-1 and it launches in second or third you have a sticky shift valve or governor plug in the valve body.

C'mon now diagnosing is for geeks, real men guess, spend lots and lots of money doing so (which supports the economy) and rarely have success. LOL
Thank You and Kunkel & John RR for your contributions to the site on "Automagics"

The gray on the dipstick -

That’s worth the effort to drop the pan alone.

Good luck, hope it’s something minor.