It's quite easy to accidentally put a timing belt on one tooth out. I take it you have the 300TDi engine? The belt is usually very tight to replace and I fit the belt one tooth out on the cam, which then pulls it back into the timing position when it's tensioned. If your engine was fine before the cam belt was done, then this is likely to be your problem.
This thread is the replacement of the timing belt as a kit. Part of it is missing though - with the cam and crank sprockets on thier respective timing marks - you may need to slacken the three central bolts in the injector pump sprocket in order to fit the drill bit/setting tool. Excess play is then removed by the belt tensioner. The timing will then be as set by the factory.
Les.
