Sometimes I need a jump start but cant seem to find cause of problem.

Problem has been intermittent for 6 months. Runs ok most the time and then out of the blue nothing. a jump start always starts it and I may have the problem for a day or 2 then gone for months. Put a new battery on last week with higher (750 amp) cold start after the van needing jump starting every time - worst ever. Started first time every time with the new battery until yesterday when I needed a jump start again. This morning starts first time, battery reads 12v and 14v with engine running and lights and blower on so i'm guessing the alternator is ok. Bit stumped.Could it be the starter motor? maybe it gets jammed and needs an extra bit of power to turn it? The vehicle is an LDV luton van with a ford diesel engine. I think it has a dual mass fly wheel and not sure if this has anything to do with it? Any help would be appreciated as I ended up getting an artic lorry tow start me once and its becoming very inconvenient.Thank you.
Asked Oct 08, 2013

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