This has thankfully only happened once, several months ago, and not repeated but I'm curious if anybody else has experienced this. I was driving down the fwy with half a tank of gas but suddenly noticed that the fuel was indicating full and the the temp was in the red! I pulled off at the next exit and stopped in a parking lot. The engine appeared to be fine and I'm sure I didn't create any new gas while driving (the car didn't anyway).

So I turned the ignition back on and the gauges returned to almost but not quite the same positions. Turned it off and back on - they again went to about 90% of where they were. I repeated the process till the gauges appeared normal and I've never had the problem repeat.

Anyone know what happened?

 

Not sure if this will help but FWIW, I've noticed that if I'm driving with the top down and the sun is shining directly on the main gauges, I'll soon see problems similar to yours. I first noticed it with the speedometer sticking. It would not return to lower indications and every time you accelerated the increased speed would be added to the old "stuck" speed and pretty soon it was indicating 90MPH while I was pretty sure I was going about 55MPH. I repeated this enough times to be sure it was the heat of the sun causing the stickiness.

Anyway, going to a dead stop the speedo would read 35MPH. Turning the engine off would cause the speedo to drop 5MPH and cycling the electronics on and then off would repeat the process until it read zero again. I've seen this with the gas gauge as well but not the temp gauge.

Now when the sun is shining on the gauge I position my hand on the wheel to provide some shade. The problem has never repeated itself since then.

 

My top is up and welded so the gauges don't see much sunshine

 

This is pure speculation, but the cluster of guages and illuminated warning lights on your instrument panel all receive electrical or pressure readings from sensors throughout the vehicle. The one commonality is that the wires to feed the guage readings are funneled through the Fascia Wiring Harness and primarily two pin connectors (Major Instrument Pack with 48 pins and Minor Instrument Pack with 24 pins). The pin connectors are behind the instrument panel cover and are on each side of the steering column. Both are well grounded. Possibly a loose ground or the pin connectors may need cleaning.

 

I would not lose too much sleep unless the problem happens again. Any computer hardware is susceptable to unlikely but possible 'soft' errors on rare occasions which may never repeat.

I have had a Mercedes C320 for 10 years. About 8 years ago, when driving along, it all of a sudden started spouting off just about every fault imaginalble on the display screen. I pulled over and restarted the car, and the problem was gone, never to recur again in 8 years and 100K miles.

 

Ya know I looked in the text to see if you had a convertible but somehow ignored the photos! Sorry 'bout that chief!

 

Unless it happens again I won't worry about. Thank to all for the comments.