What OS are you using? I have also had this problem. My forms work fine running on NT, but on 2000 I get some strange behavior with combos. I'm using a tab control and when I switch tabs then switch back, I get highlighted combos and combos reverting back to strings that I had previously truncated in my functions. Sorry I don't have a solution, but if anyone else has had this problem I would like to know.
Shanti