When you take the oil filler cap off you introduce a vacuum leak which will normally cause these to cut out.. if it 'fixes' your problem, then that's a pointer to something else that's wrong.

Your turbo isn't overheating, they're supposed to glow (when ragged).. If your timing has slipped seriously and its retarded your spark timing, then perhaps that'd cause unnecessary glowing (eg, under normal use)...

I'd get a qualified mechanic to look at it.. preferably someone that specialises in these cars.. the turbos dont require the water cooling.. (although losing all your water from a broken pipe isnt going to help your engine).