i have a 1998 XK8 and the cruise control works when it wants to and will trip a warning light on the dash about the engine warning limp mode, only it runs fine, and once i shut the key off, it resets and all is well again until i use the cruise again... anyone have any ideas??????

Probably a vacuum solenoid, you will find them in your righthand wheel well, door end, around $50 each and thats from Jaguar..... In the UK that is lol