Can't say I've never had a freeze up, but they aren't common, for me at least. In building campaigns I run on autopilot alot at X16 speed while testing stuff and in my experience launcher exe crashes are more common (and those pretty rare now).
It's always worth checking temps while running game? (using something like CPUID HWmonitor)
You have a new graphics card- did you
completely remove all graphics drivers before installing new card?- usually a good idea
The freezing- is it always the same mission/place in mission?
If it is let me know which mission(s) it is and I'll have another look to see if anything has changed in 1.08 (or are you running these on 1.05?)
PC certainly has a bit to do with these campaigns- they both like 100+ AC for some missions
Personally if this happens to me I refly the mission (maybe going to normal speed earlier- if you must use time acceleration )