Very hard to start
 

1989 Benz 260E (103 engine).
Very hard to start when stopped for 3 or 4 hours. Not weather related, hot or cold, no difference. It takes a good 10 seconds of constant starter operation before the engine catches. Once started, it runs like a charm. No hesitation, fine idle.
So far I have replaced:
Spark plugs (non resistors), rotor, distributor car, spark plug wires, fuel filter

If the engine's warm after sitting say for an hour, it starts right up.
I'm puzzled. Any suggestions?
Thanks

Ya DAba Do
 

take a look at your fuel pressure regulator, disconnect the vacuum line and see if you smell fuel in it, you may be loosing the diaphragm in the regulator
bosh has allot of regulator problems.