Mine began not starting as the weather changed and the days got hotter. I would return to the car to leave for work and it would not turn at all. Stereo and lights worked fine.

Thought it was the code/security but finally took it to the shop and they said it was a loose connection on the starter solenoid. 80 bucks later fixed NO MORE PROBS!

Good luck!

Tom

Did you check the starter solenoid? This could lead to the exact situation you're describing. Oh be careful, the starters are a pain to get to in 6ers.
My 88 635 CSi wont even try to start when hot, it doesnt even turn over. Leave it for 90 mins or so and it starts fine. It will usually start ok within 10 mins of stopping whaen hot so getting petrol is ok but if I want to stop somewhere for lunch I have to make it a long one! Strangely jump starting from another car will start it. Its not the battery or alternator, both tested fine and the battery was renewed in March 01. I suspect the imobiliser (Cobra)but the supplier says its fine. Any ideas or suggestions welcome