Unauthorized Gear Change?

I have had an unusual "problem/occurance" with my '98 323is where intermittantly it will kick itself out of gear. This occurs very randomly and only happens in first gear. Most likely scenario will be I pull up to a stop, put the car in neutral for some period of time, then put it in first and start to let the clutch out. As I'm doing that the gear shifter will violently pop out of gear and make a loud (soil your pants loud) clunk and I'll be in neutral. Again, it is intermittant and in order to avoid it (with some repeatability) I will cycle the tranny in and out of first several times before attempting to let the clutch out. This seems to avoid help and I haven't had it happen if I cycle it in and out of gear. Any ideas as to what is happening? Do I need to adjust the clutch? Is it time to replace the clutch (110,000miles)? Help! I'm running out of underwear!

If you don't have problems changing between other gears, then it would point to the gearbox itself! Sorry.

Stephen