I've been trying to find this movie from when I was a kid.
The only scene I can remember is this guy diving down a highway and these huge street light sized tentacle things sticking out of the ground trying to swat him.
I think this movie would have been made somewhere from the 60s to the 90s...
most likely its the speed selector switch. Replacing it would probably solve your problem. High is the least used position and gets the least abuse so its usually the one that works the longest, you might be able to confirm this by selecting a setting that doesn't work and jiggling the switch...