Lock up not staying locked up

[legacyelectric]

I have a 1992 2500 150,000 miles on engine, the 4l80e was rebuilt by a back yard mech. some time ago I don't know when.

Anyway the tranny when driving at highway speed pulling a trailer with a small pickup on it in od (65mph) rpm is about 2500 and from time to time it feels like the lock up is going in and out and the service engine light goes on and off at same time.

So I slowed down to 55mph and put lever in 3rd gear and it the lock up did the same thing, but after a while going down a hill I shifted back in od and it never did it again the rest of the way home 100miles.

I have noticed it doing it a little not pulling a trailer since that time
Is there a seliond that is going bad? What about doing a manual switch that is talked about here for the lockup?

I don't know much about what to do for that mod any help would be appreciated.

Thanks

Larry

[DmaxMaverick]

You need to read the code and DTC history. If not, you won't know if the DTC is causing the unlock, or the unlock is causing the DTC. Other DTC's can/will cause the unlock, among other things. It may not be a tranny issue, at all, and you won't do yourself any favors trying to throw parts and guesses at it.