I noticed that you were using Missing Sync before without any problems. Have you introduced anything new to the Treo 650 or Missing Sync that could be the cause. I too and using Missing Sync.. having great results with it. So, I would...
- see if you've installed any 3rd party app's or utilities that aren't getting along with your Treo
- change the settings in your conduits to concentrate on just the core synchronizations (contacts, address)
Sounds like it's more a Treo thing than a Missing Sync or Mac issue.