lost rpm

[dav70]

i have a 2007 df60 that was fitted in dec 2011 it ran fine all 2012 up untill 3weeks ago we go out divin it takes 1.5 hrs to get there it runs @5800-6200rpm we dive for 2hrs then come home 5mins into the run home the rpm drops to 3800rpm at full throttle it has been over to the local dealer an they keep saying water in fuel but this has happened 3times now an each it goes to the dealer i get the same answer water in fuel but i have drained the tank an flushed it every time this time i have the owner of the petrol station come over an test the fuel with a water paste an there is no water it holds 3800rpm all the way home i am baffed an feel like i am being fobbed off can anyone help