2002 f80 tiller high temp light

[jjeyeguide]

My high temp light started coming on, talked to a couple people that said a week impeller will could cause low water pressure and be the problem. I replace the impeller and still had the issue. As I put a load on the engine the light comes on and the minute I return it to idle the light ussually goes out. I took it to a dealer and he has run it for two days on amuffler from a hose and has not been able to duplicate it? Any help would be appreiciated.

Thanks,
john