too hot only affecting mid range?

[Serum]

Sounds like a problem with the speedo to me.

Which is weird, because it gets small pulses from the receiver, the length/speed of the pulses only change from half WOT to full WOT..

Did you checked your capacitors on the speedo? check them to see if they are not popped.

You could test another battery set if you have it. Sounds odd though, i can't imagine it are the batteries, but you'got to aim before you shoot, right?