Lots of people here had bad experince with Itp ITPS powering PC and monitor so you may want to double ckeck that.
Other problem could be heat.... So maybe with a little more inpu from you we can help you better...
1.when you checked the itps what number are you geting??
2.there is any source of heat dissipation in the car pc i.e thru fans or a/c
3.this only happen when you are driving or when the engine is off also???