Speedo Variable speed Resistor

[bluebottle]

I have had an ongoing problem with my speedo reading incorrectly.
If I move the ECU wiring loom and reposition the ECU (give it a shake and put back) the speedo reads correctly, and keeps doing so until i have a passenger then it goes all Pete Tong again and reads about 30% lower at all speeds ie 50 when I'm doing 70mph. Kinda got used to driving on the engine revs/sound and the Road Angel.
But yesterday after fiddling with the wiring loom and repositioning the ecu I got it working accurately again...until i switched the heater on?! now every time i switch the heater on or off i can induce the 30% speed variation.
Any suggestions on what is causing it? I cannot get it to replicate the fault when rattling the wires only when repostioning the ecu (which i wedge under the fuse box) Also the Rev counter has always lied usually reads about 1k over (when it stops bouncing long enough, which only seems to happen on full throttle).

[bluebottle]

:Bounce:

Any suggestions chaps?

[dnb]

Sounds like a grounding issue to me.

[david beer]

There is a converter behind the dash, maybe you are disturbing the supply to it ?(well there is on my car!)