When ConvertXtoDVD crashes, there's a ConvertXtoDVD_Crash.log that exists in your
C:\Documents and Settings[USERNAME]\Application Data\Vso
BUT Do not restart ConvertXtoDVD, since it will delete that Crash.log and create a fresh one.
I have a feeling it's a heat problem as you mentioned.
Try a convertion with the TOP removed and/or with a big fan blowing air on the CPU/Motherboard.
Good Luck !