I've been searching through the piles of threads on 'overdrive' issues, but not sure that I've found quite this issue:

A 1989 740GLE (wagon) 4 speed manual, just over 200K, with 'electronic overdrive' 5th gear. The car runs fine, though I've only had it a few weeks. The over the course of my time with it, the success of convincing the car to engage the 5th gear has declined sharply.

In 4th gear, I'll press the overdrive button on the stick...the "5" light will come on the dash...and the car will happily remain in 4th gear, the shift light screaming at me to relieve the 3800+ RPM howl.
When I first got the car it would shift almost immediately after switching.
Now, there's no rhyme or reason: sometimes it will shift, sometimes it won't. Clutching before shifting, not clutching, revving in various patterns, accellerating hard, decelerating, cold engine, warm engine---all to no avail. Sometimes, after a while 30 minutes or more, it'l shift randomly. Sometimes it won't at all (which is miserable for fuel economy on multi-hour trips across the state)
Always, the "5" light comes on...something in the car recognizes that I *want* to shift.

There are some other gear concerns, I won't go into them now... Can anyone offer any advice?