Classic problem of the 300E. I have not yet heard of a 300E that does not do that (although I am sure there must be some of them hidden somewhere). Mine does it and everyone else's I know of does it too. The prevailing notion seems to be that of vapor lock. Personally I do not mind and decided to live with this minor annoyance, but if you want to fix it, there are many possible reasons some of which are:
leaking fuel distributor plunger,
leaking pressure regulator,
faulty one way fuel valves on the fuel pumps,
faulty cold start enrichment valve.