Tapehead,
That's really interesting.
I wonder if the motor or flywheel might be slightly un-parallel to each other. Any un-paralellism would tend to make the belt want to 'walk' off the pulleys. I think if the belt is too tight, it may possibly pull the pulleys slightly out of parallelism, too.
I agree with you, that it would seem very annoying. The solution of leaving the cover off, I'm sure is 'not pretty', but true, it's effective, but be careful it doesn't become a safety hazard, especially if you have little kids underfoot. The flange idea sounds good too.
I have Fostex's A8 and
Model 80, as well as Tascam R-R's too. I've worked on both, under the covers. Once under the covers, I think the Fostex's transport reveals itself to be cheaply made, and definitely not on par with Tascam's transport. The Fostexes are functional, but the Tascams are definitely more rugged, more well made, and of a slightly higher caliber of engineering. That's just FYI, and no offense.
That being said, I hope you can resolve this flaky belt issue. Maybe report back to us if you find anything significant. I'd be curious to know.
No doubt, it's quite an unusual problem. Most of the time, belt issues are pretty straight forward and easy to resolve, but this problem might go beyond the belt.
PS, if something's out of parallelism, or askew on your Fostex, you might try to 'shim' it, a little at a time.
Good luck. DA