audi S8 1999 AQH 360bhp overheating problem

hi,i have a 1999 s8, The engine code is AQH. The temp guage sits at 90 then rises to 100 then back down but just goes up and down from 95 to 100. I have replaced the viscous fan, did the carrot test and the fan just stopped, Replaced it with a new item from euro carparts, replaced the electric fan as this wasnt working at all also replaced the two temp switches,, audi replaced the timing belt,waterpump and thermostat etc 2 years ago at 120k, mileage now showing 155k.The electric fan works when the ac is switched on, but when the heaters are on the fan kicks in, but if i turn the heaters off the fan turns off is this right? im not sure what else to check now but im assuming maybe my waterpump could be gone.

help please?