My BT7 (with a BJ8 gearbox) had the same problem a couple years ago. It would engage but after 5-10 minutes it would disengage and not reengage. I checked all the usual suspects and then came across an article from 1985 that identified the problem. The source of the issue was a worn non-return valve. A weakened non-return valve does not allow pressure to be maintained for proper OD operation. The OD works when cold but once the pressure and temperature increase, a worn non-return valve can't hold the pressure as it should. The plunger, spring and ball are available through Moss (#329-510; 866-070; and 866-080) all for under $20. Replacement can be done by draining the OD oil, removing the gearbox cover, solenoid, and its mounting bracket. The bracket pre-loads the accumulator spring so take your time and follow the instructions in the shop manual. Note the location of the spacer and rubber o-ring on the back side of the bracket. You should now see the hex head of the non-return valve plug. Remove the plug (may be very tight) and then the plunger, spring and ball should come out. Replace all three parts. (In my case, the plunger was worn and comparing the compressibility of the old spring and the new spring revealed a discernable difference.) Assembly is the reverse of removal! The entire process took less than an hour by a true amateur (me!) and I have had no further OD problems in the 6 years since I made the repair. For the small investment of time and money, this fix is worth trying IMHO.