TH350 kicks out of 3rd gear

[oldgold70c10]

I've got a 1978 Big 10 with a TH350, 76k original miles. Truck had 60K miles when I bought it a year ago. truck has been babied its whole life. It had 48K on it when my friend I bought it from got it from his uncle in 2007, who owned it since in 1980.
Lately, the transmission is starting to act up. After it is warm, when running in drive, it will go from third to 2nd or 1st on its own. Quickly moving the selector to neutral and back to drive will put it into 3rd gear till it does it again.
Today, these symptoms started while the truck was still relatively cool.
Fluid level has always been normal, no leaks. I changed out the vacuum modulator with one from NAPA with the red mark. Made no changes. Fluid smells burnt. Should I try a fluid and filter change first, or bite the bullet and have it rebuilt?

[Bigdav160]

I'd pull the governor and check the gear

[oldgold70c10]

Originally Posted by Bigdav160

I'd pull the governor and check the gear

Thanks, I will give that a try. Earlier when driving home from work at highway speed, when it would kick out of 3rd, it would actually go back to 3rd on its own before I could react and shift to neutral and back to drive, at low speeds it is erratic about whether it will shift out of first gear on it own, or after shifting to neutral and back to drive.

[68Gold/white]

Had similar experience with my 77 Trans Am.I have owned car since new. Original TH350 has never been out of car, although has had a 1970 Pontiac 359 engine since 1992 (was supposed to be a temporary engine, is still in car, but not much longer...)

3 different times, I was driving on highway, transmission felt like it just fell out of gear. It was in 2nd, you could get trans to pull in 2nd, but RPM's went pretty high. This was about 16 years ago, it has not done it since, although I seldom drive car.......

A co-worker had been a trans mechanic at a dealer for years told me the rubber seals DO get old and hard, and that some hub was over riding something (because of clutch wear) not allowing trans to stay in high gear...

I would get trans rebuilt.....it's due, so is mine...

[oldgold70c10]

Thanks guys for the replies.
I pulled the governor Sunday. Gears are in excellent shape, weights and valve spindle inside governor are free.
Checked the kickdown cable too, because that is what it is like at highway speeds, like someone randomly yanking the kickdown forcing it to drop down a gear. Kickdown is free, snaps back when pulled.

I guess it is time for a rebuild.

[YoungPup1977]

Most likely your Direct clutches have to much clearance (clutches are worn). Th350 are known for the direct drum (bushing) to wear out the stator and create pressure loss into the direct drum. Make sure a wider bushing is installed in the rebuild. The pic shows a narrow (factory) bushing. Good luck!!