sounds like (if its not the glow plug) you have a slight air leak somewhere that cures itself after running for a few seconds, then when the engine begins to heat up it works ok, have you dismantled the engine? maybe a new head gasket is in order... put it on the compression stroke and see if you can hear air escaping from other than the muffler, glow plug tight? glow plug washer ok?...Rog..also may be leaking around the backplate or carb?..