XK150 Automatic Transmission won't stay in top gear

My Borg Warber automatic won’t stay in top gear. It works great when cold but when it warms up it sort of meanders from top gear to second on any acceleration or load. I’ve had the tranny rebuilt and my guy has had the back section out so many times I’ve lost count. It’s so disappointing to think I am getting somewhere and when it heats up back to square one. I thought it may be the top gear bands so I’ve tightened thos up - nothing. I had them too tight at one point and the car wouldn’t more. Any band tightening pointers? Any ideas what I can try next?

If you have the original DG box, I beleve it is a 2 speed unit and top gear is obtained by a direct drive clutch in the torque converter. IIRC, there is a cylindrical tube that goes in the front of the gearbox input shaft that supplies oil from the gearbox to the clutch to engage top gear. This may be where the issue is; I had a Mk2 where the mechanic forgot to fit the tube after a trans o/haul and top gear would not engage. If there’s any pressure leakage around that area, that might explain why it works when cold.

Thanks. That tube is definitely installed.

I’ve removed the back pump from a parts transmission and going to rebuild it to specs (my friend is a machinest) and give that a try.

What type of oil are u using, only ATF A or ATF F will work.
Any Dexron will not work, as it is to slippery for these old boxes.

Thanks Jan. I’ll certainly give it a try. I have a spare tranny and my friend is a machinest. I removed the back pump and we are about to rebuild the pump to spec thinking prehaps the pump in the car is loosing pressure when heated up. When the car is cold it works great. When it gets to temperature it glides back to second at any load at all. FIngers crossed.

Question - will draining the existing tranny and torque converter tranny and adding ATF work?

There is also a drain plug in the rear extension housing.
Yes a refill could help, do you know what kind of oil there was in there?
There will go 8.5 l in there.