5th gear problem

I recently encountered a problem with 5th gear on my '79' R100RT. The gearbox had an overhaul 1 1/2 years ago when I had major repairs done to the engine top-end, gearbox and FDU. This last weekend I had gone 2 hours in 5th gear at 120 km/hr when the rear wheel gave a small chirp accompanied by a momentary rpm bounce down to 3000 rpm. Nothing else happened for 15 minutes when the m/c started surging with successive rpm rapid drops. It felt like water in the fuel or carb icing, but when I pulled in the clutch the engine reved just fine. I freewheeled to a stop to do an inspection. After starting up I went through all of the gears to 5th when the problem started again. I dropped down to 4th and put another 400 km on the bike until I got home with no problems. I was a bit afraid to test it in 5th again in case 5th gear would stop the rear wheel. It seems to me as if something has come loose involving 5th gear and tries to grab something when 5th is engaged. Has anybody out there seen this before? Thanks to everyone for their suggestions about my brake bleeding problems. They didn't work originally but I just recently retried them with successfull results.

Uh Oh..I've been similar (& hope UR not)

If you have: all of 1st, most all of 2nd, up to ~ 3k of 3rd, all of 4th, but "ZEEEK" in 5th: you lost a particular tranny cluster gear. My '76 R90/6 suffered my ignorance in said manner... I (1st BMW) didn't know tranny oil was separate from engine. Contact your resources should this be the case, sorry & good luck... <<<)))

Thanks for the response. Gears 1 through 4 work totally normally. The roughness in 5th, almost like it is snagging something, seems to be intermittant. The first time it happened, it reoccurred immediately when I went into 5th gear and I didn't try 5th again. Later, a BMW mechanic test road the bike and couldn't get it to act up again. He felt that the problem might have been the rear brake, which still wasn't working at that time, and might have been lightly rubbing the disk, causing it to heat up and grab the rear disk. However, in his personal experience that resulted in slowly increasing pressure on the disk until it the wheel wouldn't turn any more, until the pucks had cooled down. Those symptoms were totally different from mine. I can't afford to get into the g/b right now so am hoping someone has seen the problem before and knows what it is so I won't go into the g/b un-necesarily. However this is a faint hope.