87 528e start issues

My 1987 528e starts and runs perfectly.....sometimes. occasionally it will crank but wont start. If I wait 1/2 hour to 2 hrs and it starts up and runs great again. a mechanic friend (not a BMW mechanic) says probably a fuel pump but I believe I have both an internal and an external pump and before spending $300.00 on a "sounds like" diagnosis was wondering if anyone could shine a little light.

Give the main relay (closest to radiator 5 prong) a check im kinda new to it all with my 84 528e ive got issues and just today ordered a cheap working dme/ecm went over alot of stuff and have a pretty rotten engine harness too, oh and just ordered a main relay myself. Guess she's gonna be down for a few days, bummer. Crank sensor can be tested as well or swaped somewhat easily(grey connector, closest of the two towards engine i believe). Hope anything helps, I want to drive my bmw.
It seems to do wonders soldering up the dme/ecu for alot of people if you want to do that sometime too.