Could be a couple things-
Fuel filter is slightly clogged allowing the carb bowl to very slowly run out of fuel.
Coil is heat sensitive and starts to lose spark when hot.(clean cooling fins)

You might state what this is on.

F525 with 18hp Kawasaki.

I should have mentioned that when the blades are turned off the mower runs fine. It gets sluggish when the blades are turned on after the mower has been running a while.

You could have fun with this one. The assumption is that this is engine related. Like all assumptions... It is best to start simple, thorough and cheap on a problem like this. If you haven't started w/ the basics- new spark plug, fuel filter, dropping the carb bowl at least and give a clean and inspect etc now would be a good time. Ignition igniters are famous for creating hot restart issues, but a loss of power- I'd not had myself. Check valves in these old metal fuel pumps can start to falter anytime. Cleaning the cooling fins is a real good idea as Bill indicated as well. Inspect closely for signs of head gasket leakage.

After checking the engine issues already mentioned, check the condition of the blade spindles and the idlers on the deck. Looking for problems with bearings.