Mike:
We have had this problem, and I do not think it is heat related. The tach is a wiring problem most likely. Look for the wiring lead, on the engine wiring harness, that goes to the tachometer sensor. These wires are two press on fittings that press onto the 1/4" flat sensor leads. Pull these leads off and with a pliers, simply press them tighter, making the fitting tighter. Be careful not to over flatten or tighten them, because then you will not be able to press then onto the sensor fitting again. No big deal, because you can always cut then off and crimp on new press on fittings. Clean the two copper tachometer sensor connections, where these wires press on, and you should be good to go. I bet this is your issue, loose fitting tachometer sensor wires, nothing else.
Hayden
