2007 DF50 temp warning 4000rpm

[Jimschush]

During a trip out yesterday on Windermere my DF 50 engine temp warning alarm/warning light activated. It runs fine up to 4000 rpm and upto this speed pumps water strongly, at exactly 4000rpm it stops pumping water and the alarm activates at which point I come off the revs, engage neutral and switch off the engine. The engine can be started immediately with no alarm or warning lights and runs smoothly with no problems. The problem only happens when we hit 4000 rpm. We are due spend a week on the lake in early August so if anybody can shed some light on my problem it would be appreciated.