engine temp

Hullo all
got a 2.5td been running a bit hot lately. normaly the needle stays pretty much straight up but its about 2 thirds across now. strange thing is it dont do it all the time i can sometimes drive for an hour with normal temp and the next hour running hotter. not losing coolant or any more oil than usual...
Any ideas?
Cheers

bump...

is it in normal driving? or under load? climbing hills etc. i say this as i had a problem with mine getting hot on hills , loss of power and temp rising. timing belt change sorted it out as injector pump timing was slightly off. worth a look fella.