intermittent hot start problem
 

my '79 300sd always cranks and starts when cold, but occasionally, when hot, she not only won't start, but won't crank at all, i turn the key and nothing happens, though the dash lights come on (the glow just flickers or doesn't come on). if i open the hood and then shut it, causing vibration, it will sometimes start again, but sometimes i have to wait a half hour or so until she decides to crank again. then again, sometimes she will go for weeks without this problem, so it's hard to diagnose when it only happens occasionally.

any ideas?

Pull the brown vacuum line off the shut off switch on the back of the IP, next time your in the "no start' situation. Could be the ignition switch is faulty and wont allow the rack return to the start position.

Are you sure your starter motor is turning fast enough when its hot (100 rpm)?