Hi Michael,
The subject has already been on the board a couple of times.
Here's one that might contain some information which may be helpful to you;
http://www.a7a10.net/forum/index.php/topic,5800.msg39710.html#msg39710(Muskrat posted an interesting technique on this thread
)
The chainwheel (I suppose you mean the big chainwheel from the primary chain?) should be be tight, and not wobble at all.
Could be that the taper on the gearbox mainshaft and/or the taper of the clutch centre has worn out.
If it's the gearbox main shaft that's wobbly, probably the gearbox's bearings have worn.
I'm sure someone can add some more horror to this?
Anyway, have fun
Patrick