Thoughts about intermittent no reverse

[sfcl]

I'd be very interested to get the opinion of transmission or BMW experts about this one as I've got puzzled: I had already explained in a previous post that my 120 000 miles 3.0I (GM transmission, never flushed) had intermittent no reverse and that I'll try to replace the related solenoid as advised in another forum. Even though I have not had the time to do it yet, I've tried to understand at what moment exactly the fault occurs. Here what I noticed in the last weeks:
- The tranny works very well in any circumtances, very smooth and accurate, no slip at all, strong reaction, etc... (I admit that this is a reason why I hesitate to change the fluid...)
- In "R" position, it works perfect most of the time, no slipping at all either and pulls strong.
- No problem with reverse at any time if I'm doing the usual drives to go to work and back, say about 30 minutes per travel, whatever the kind of driving or transmission mode (usual or sport).
- The no-reverse occurs about one time every 2 weeks, cold or hot.
- I have just to drive the truck fast for 30 seconds or 1 minute to clear the problem and get reverse again!
- I've noticed that every time when I got the no reverse it was after a longer drive (say 1 hour) at slow speed (say 45 to 55 mph), being very careful and soft with the tranny (just like when we are gently travelling with my family on any Sunday). On the other hand, if I'm driving the truck harder, getting hard acceleration and kick-down, I never get the no reverse problem!...
- In the case I get the no reverse after a long "soft" trip, it occurs either with the engine hot just coming back home or the day after when the engine is cold...

As I'm not always in need to drive fast and hard , I'd like to understand what can happen in the tranny while driving softly... Would it be that the fluid is so dirty that it can clog a solenoid mesh, and that the mesh clears itself if I drive fast and the oil pressure is stronger??
What would be your opinion about that mistery?