wm6.5 exchange certificate problem

[Gspt2000]

I have a HTC Touch Pro 2 with WM6.5 that does not keep the certificate for my exchange server. I install the certificate into the phone and setup the exchange settings in activesync, and it works fine for a while. After a day, activesync on the phone or desktop says certificate not valid. Has anyone run into this type of issue?
I have Windows Small Business Server 2008 with exchange, and a self signed certificate. My partner has NO PROBLEM with his HTC Fuze (WM6.1) and the same settings!

Any information on this issue is greatly appreciated!!!

Thank you for your help...