2006 Ford F-250 Temperature gauge pegs out

When it's hot and I'm driving with AC on, sometimes on those especially hot days, the temp gauge goes to max and I get a check gauges warning. I turn the AC off and in a couple of minutes I get normal temp reading. But it's not gradual. It's normal or max all at once. Either on the way up or down, no gradual readings. Normal or hot. Mechanic cleaned the radiator exterior and replaced the thermostat. Got better but still happens on occasion. No leaks or lost fluid and doesn't seem to be overheating. Was thinking temp sending unit, but Ford wants $75.00 for it and I don't want to experiment at that price.