Actually, changing the timing belt on the 928 (and the 944) is a very difficult process. Just to get to it you have to remove the fan and throttle linkage, alternator, compressor, smog pump, distributors, rollers, pulleys, and the water pump -- plus the tensioner and 2 belt covers. Once you get the belt off and replaced you need a special porsche tensioning tool to tighten the belt that costs $500 (a knockoff is available for $125 online) ... Any 928 guy worth his salt will tell you that you should replace the waterpump and rebuild the tensioner at this point (since you have them off already) -- Waterpumps and tensioners are in fact a big problem on 928's ... you don't necessarily have to do it at the time of the timing belt change, but chances are you will need to do it before
another timing belt change. The guys that know these cars recommend 4 years or 45,000 miles for a timing belt/water pump/ tensioner service... I tend to agree with them and for those of us "morons"

out here who might not be able to do it ourselves, that's a $1500-$2000 job.