My development machine are running Win7 and using an FTDI USB<->UART bridge connected to a old resler RS232 interface. Today I was pushing it in almost an hour without any problem at all.
The Car PC are using XP SP3 and the onboard Com port connected direct to Ibus interface. This config has been running almost a year now with no performance issue at all and last week I did 1570km in 2 days with one 7hours non stop drive - no problem at all.
One idea is if you using the newer V6 or V6a Resler interface with Silicon labs bridge since there are some bugs in the older drivers that could cause the problem. That's why i ask what interface and driver version your using.
So pls check what interface version you have and drivers.