e34 first gear problem

Hello,
I have a 95 525i with 170k miles original clutch and transmission. Lately I started having problems with my first gear. When I shift in to first gear and let clutch go, car starts to move and then kicks out of the first gear. I am not really technical person so please bear with me.
It does not happens all the time just once in a while and I always try to make sure that I completely shift in to the gear. What could cause this kind of problem clutch or transmission?

Thank you for the help.

This was also one of my problems with my 90 535i. (see clutch conversion query below). I replaced my shifter seat bushing and it helped alot. Make sure other parts of shifter linkage aren't worn. On my car, the problem is still there but I can feel the resistance going into first and also clutch drag when moving shifter out of any gear at a stop (flywheel problem).
You are correct to suspect the clutch and possibly the trans itself. Hope this helps.
john