Not a huge issue yet, but during the last two summers, when the car is parked outside & the interior heats up, the tachometer will rise from 0 to between 1 - 2000 rpm and just sit there.

This is with the car not running, with a shade for the dash / front windshield, and usually occurs when it is hot weather, high 80's & 90's.

Driving the car, the tach will respond at higher rpm's but will not drop below 2000.

Looks like a sign to me that the tach is getting old & needs a rebuild or replacement.

Anyone else experienced this or have any insight.

I replaced the electronics of my '78 tachometer with an aftermarket board made by rt-eng (RT Engineering) for Mopar tachs. They're sold on ebay, for instance:

1967 1974 Mopar Replacement Tachometer Board | eBay

I had to disconnect the meter movement from the board and solder connections to the board, which I mounted on the back of the tachometer inside a tuna can (for a shield/housing). The meter adjusts fine to the Datsun I6. I suppose $80 is a bit of money, but at least your tach will be accurate and will last into the future.

I've had the heat-related needle sticking on a running engine, and seen the same from others. Haven't seen a fix yet though, for the original equipment. How about a cooling fan for the gauge? Typically, when it's hot out the gauge will follow RPM up but won't come back down until things cool. I have noticed though that my tach sits 200 RPM high occasionally with just the key on. I'll have to watch the heat and see if there's a correlation. I've never heard of one that just rose on its own with heat. Is the key On? Where would the electrical impulse come from to get it started?

I ended up swapping a 78 tach in to my 76. Font mismatch!

Thanks for the info on the replacement board FastWoman. I will keep that option open for the future should I go that route.

Zed, the ignition is off, and it only does it when the interior gets very hot inside. I think it the spring movement on the tach needle being expanded by the heat. More of an annoyance than a problem, its been hot in Seattle the last month or more.