Cave Explorer

Temperature guage is flucuating between high and low in the span of about two minutes as I drive down the highway. Doesn't exactly overheat or go completely cold....Just covers a good span in the mid range. But clearly unstable. The water pump was original (1996) so I figured the impeller or bearing was shot. Replace it and the thermostat. Same problem. There is no coolant leak visible. Engine combustion was checked but it did not show a 'fail'. However I suspect there is nevertheless an internal coolant leak through the head gasket. Anyone have a problem like this? Thinking of using Steel Seal.

At nearly 300K miles, the original exhaust gave out. Replaced with quietest Walker aftermarket muffler but now have annoying throaty drone that original never had. What is best option to get back to oem quiet level? Silencer? Resonator? different muffler? Catalytic Converter replaced 2 years ago also with Walker. Aftermarket CC is a little smaller than original so I'm thinking this may have something to do with overall increase in noise. Not wanting to go crazy with $ to fix problem but do NOT want to hear exhaust.