ldv convoy starting problems?

I have a 1999 convoy tipper with the 2.5 transit banana engine in it. when i turn the key it struggles to turn the engine over. I have replaced the battery, put in a new starter motor and replaced all wiring from battery to starter and earth cables to. if i put jump leads on battery it makes no difference, if i put them on starter and engine it turns over fast as hell and starts first time. i'm pulling my hair out. Any help would be greatly appreciated. oh, once started it runs sweet all day, never misses a beat. thanks, Al