I would do as Griff suggested, is it the same if you are in the water? If you are sucking in good water flow, then I would check the hoses connected to the T-stat , are they bent so they are restricting the movement of the poppett balls? If I was a betting man I think that the circulating pump is not working right. You have temp problems and the belts on the pump dance around sounds like the two maybe related? Pull that pump off and look at the impellers and check the bearings. l'd take a good look there. Good Luck

I use the KISS Method when I can't figure someting out!

K eep
I t
S imple
S tupid

Oh that sounds easy enough.......I gotta pull the drive apart to do it...... It does the same thing in the water just less noticible......

I have never let it purposely get too hot.....just adjust the throttle accordingly.....call me weird but it's like a reflex.....

The check balls DO NOT seat all the way on the housing....this I noticed......would that cause it.........

Before I put the new impeller in it did it too.......but a bit more severe............maybe the housing is shot?????

I just replace the impeller.....whatta dummy huh?

I was lucky, A merc mechanic told me way back to always replace the whole thing so i always did. As to those poppett balls ..Dunno. Never had a problem with them!?

Shor-
Definitely put a whole housing/seal kit in when going Globe- the guys with the Bravo engine-driven pumps have had problems with just an impeller change (scored liners).
After overhauling my engines, one of them didn't have any water output on the hose at all- I used the old engine water pumps (they worked when I took 'em off right?). New impeller & housing kit didn't help. I put on a new engine water pump and good as new!
There was a pulley alignment problem with the belts on certain 260's and 270's- it generated a service bulletin from Merc to install a spacer on one of the brackets. Check with your local Merc service dept. to see if your serial number might be affected and if it was ever done.
FYI, my SBC's/Alphas in the Formula act completely different on the hose than submerged in the water- I finally cut down a plastic garbage can to immerse the drive in and just stuck the running garden hose in the can. No more problems running in the driveway.
Jim

The cold water running it on a hose will make the thermostat open and close irregularly. You have to run it in the water to get a true temp reading. As far as the belts, do they squeal? Belts always look wobbly when running at high rpms. Make sure they are all in line with the next pulley. I have to agree with flylevel with the kiss theory. It sounds like your looking for problems you don't really have.