What are the possible reasons a 1985 VW Cabroilet with a new starter will not turn over or turn over inconsistently?

the wiring should be checked- voltage to the starter and also to the solenoid when the key's in the crank position. Other possibilities include battery and terminals. The "new" starter is suspect too, especially if this is an automatic VW Cabriolet. If the problem is a failure to turn over when an attempt is made to start the car when it's hot, definitely suspect the starter. There's a relay kit to remedy this problem too.