2008 T255: 'sticky' speedo in hot weather

So I drive a 2008 5dr (T255) out here in the blazing tropics. I got off work early yesterday and drove home in the heat of the afternoon sun. My speedo worked fine until I hit the motorway and accelerated to speed - that's when I noticed the speedo getting stuck at 50kph (was going at least 90, based on engine revs in fifth), where it remained until I slowed down on the exit ramp. As I slowed from highway speed, the reading dropped, so effectively it was displaying a reading about 30 to 40 kph less than my actual speed. When I came to a stop at a red light and set off again, the speedo behaved normally again.

Puzzled, I took it for a short spin later that evening and everything was normal. Everything was also normal this morning, and it read 90kph correctly as I cruised down the highway. The needle travel was buttery smooth, not jerky at all.

I was able to replicate the issue again this afternoon when I left the office to get takeouts for lunch, under the raging heat of the tropical afternoon.

Has anyone had a similar experience?