hi roaddawq is the unit a tb45 engine ? if so I will tell you about a similar experience with the same sort of fault which for some time was a puzzle it turned out that the unit had the same symptoms that you have and after trying all sorts of remedies it turned out that the hydraulic pump connected to front pulley was seizing up slowly and not allowing the engine to start and run properly.
maybe this will help if its the same engine!!!!