After the failure of my recent P800 firmware upgrade to solve certain long-standing problems, I finally splurged and replaced my el-cheapo generic bluetooth dongle with a D-Link DBT-120 bluetooth dongle, which is incidentally the dongle that Apple recommends. This has completely and utterly solved my often mentioned syncing problems. Syncing works 100% of the time now!
All Apple needed to do was to put a warning on their website - something along the lines of "using other brand bluetooth dongles may cause iSync to crap out" would have sufficed.