Is anyone on 2010 having issues with assembly mates?

I have an assembly created in '08, worked on and saved in '09, and now I'm working on it with '10.

My problem is that I'll constrain a part in three directions making it become fully constrained. I'll then wander off and work on some other part of the assembly, and whilst I'm doing that, the previous part/s worked on somehow become over-constrained. If I delete 2 of the 3 contraints, this gets rid of the over-constraining, but the part is now fully constrained with only 1 assembly mate! If I delete the last constraint, the part then becomes un-constrained in all directions!

So I then mate the part as before making it fully constrained, then the over-constraining cycle starts again!

Is this a bug as I've never come across this before?

Cheers,

Bruce.