Hi Craig
Problems with the fast forward are fairly common and I've recently fixed a few myself, however, not specifically on the BC8000. Fast forward is initially OK while the take up reel is small but everything slows down as it accumulates more tape. These seem to be your symptoms as far as I can tell. There are two areas that you might like to look at. First is the action of the clutch - can you adjust it to put a bit more pressure on the spring or can you get hold of a better clutch unit. I think that you can still get them as spares but they were a bit pricy. The second area is the small jockey wheel that completes the drive - sometimes it needs lubricating so that the spring can pull it properly into place and apply the right amount of pressure.
Have a look at both of those points, you might get lucky
Regards
Geoff