Series 60 overheating / underheating -- intermittently.

I have a 2001 series 60 that is driving me bonkers. Some days, the temp gauge will come up to temp, stay at 185 unless pulling a heavy load.

Other times, especially after pulling a hill, the gauge will become erratic. It'll read 170, but then the fan will come on, and it'll swing all the way to 220 and the overheat alarm comes on. Then it'll drop right back to 210, and usually keeps dropping down, sometimes staying at 200 or so, sometimes dropping as low as 150 before swinging back up -- in a matter of seconds.

Today, she ran perfect before lunch (dead heading home), but after lunch she ran at 170 -- for three hours straight. After a stop, she came up to 180, and stayed there.

Changed thermostats, twice, water pump and sensing unit. No changes. Some days she's predictable, some days she's all over the place.

Help!

 

We cleaned all the connections to the ECM and the sensing unit. Don't know if it has two or one; I believe it is one. When the gauge swings to overheat, the dash overheat alarms go off exactly at 225, so I'm assuming the same sensor feeds the ECM as the dash gauge.

Mechanic is going to go at it on Monday morning with the heat guns, he suspects a partial restriction somewhere. I'm wondering it a thermostat may be sticking in the housing, erratically.

 

Radiators were all taken loose and cleaned a month or two ago (external only). She cools better, but the goofy gauge readings didn't change. The fan comes on pretty much right at 210, even if the gauge reads 180, (or whatever), but I was told it works off a different circuit. Bottom hose was changed this year, probably should change the top hose, too, for good measure.

I'll have the mechanic plug in his reader to verify the temps, and chase the ground down again.

The strange part is that she'll often run normal, until pulling a hill, and after that she'll be erratic for a while.

Thanks for the suggestions