slow to crank when warm

my 300tdi cranks fine when cold,
but when warm it cranks real slow, sounds like the battery is flat,
i'm pretty sure i remember an old V8 camaro i owned had a similar problem and it was heatsoak in the starter,
do you guys think this sounds the possible cause in this situation,
cheers,
neil

coud be... or if the battery is old it can suffer from warm too, you can check this by attaching jump leads to other car when yours is hot and if it cranks better you need a stronger battery... also check the earths to be well tightened

thanks for that,
battery is brand new and I also swapped batteries (got 2 300tdi's) and it was exactly the same,
this battery was fine on other one,
earths look ok but it can't harm to remove them and check they have a good connection,
looking like it's the starter then,
cheers,
neil