It'll help the Stilo boyz if you can be more precise about what model the car is, but from memory this can be the crankshaft pulley or variator (if fitted)
Hi Paul,
Welcome to the forum, the 1.8 is known for variator issues with the exact symtoms you describe.
There have been quite a few enquiries on other threads, if you do a search you should find loads to read with diagnosing help, etc.
** From memory ** it requires the variator being replaced but please check on the older threads for confirmation.