CSC has a table with Saving Throw values for electronics against an EMP grenade. I think those could be used for radiation/emp hits against the computer if you wanted to.
My opinion (and Referee's Ruling) is that a system that has 3 hits CANNOT be repaired. It is destroyed.
As for endless battlefield repairs, I say YES, it is possible.
A battlefield repair effectively restores 1 hit, so as long as you don't get that third hit before you can repair the system, you are OK.
So it becomes a running fight between repairing a system before it takes another hit.
I don't work for Mongoose either, so YMMV