bug in the elaectrics

Hi there, got a little problem with my 1983 golf gts, start easy but all of a suden all power switch off , its like you switch off at the igntion, some times drive half an hour, somtimes lomger and sometimes, just after you start it.
If any body have a idea where to look, please help :dontknow:

Check your earth straps bud, might be losse, need cleaning etc.
Main one is from neg on battery to gearbox bellhousing