According to the temperature guage the temperature varies between 160 and 190 degrees when cruising. The time it takes for the temperature to cycle is in the order of one minute but it varies. If I increase the speed it seems to trigger the cycling. At idle it is stable at 160 degrees. The pump, impeller and hoses are in good shape and the amount of water that comes out the exaustpipes is OK. The exhaust manifolds are cool. Could it be the thermostat? Thankful for advice!

The engine is a 1999 502 mag MPI and the impeller and drive hose have recently been changed.

Had the same problem, It was chunks in the water hose feeding the engine. found most of it in the oil cooler. The fluctuation in temp is from steam pockets being created in the engine from lack of water volume. I had plenty of water pressure, but it wasn't enough volume to supply the block. The water coming out My exhaust looked fine but that's because the restriction wasn't on the exit side of the engine. so I didn't think it was that at first. But then I found old pieces of impeller and other junk sucked up into the oil cooler and cross over/circulating pump.

Same here when I went faster it moved faster. at WOT it was like having a windshield wiper blade for a needle... but at Idle you didn't see anything.