Pegueot 306 1998/S reg 1.4l petrol.

Last night, a minute or so after driving off (quite possibly first time
I checked my speed) I noticed that my speedo and rev counter were giving
incorrect readings.  Rev counter was stuck just below 2000rpm, speedo
was around 140mph (highest marking).

I turned the engine off and the rev counter returned to 0.  Speedo
stayed where it was.  Started engine again and drove off.  Rev counter
worked fine after that.  Speedo seemed to quiver every so often, usually
increasing it's 'speed' - to the 5 o'clock position.  I then turned
engine off again and this time the speedo, after about 5-10 seconds,
shook its way back to 0.  Speedo worked perfectly after that for the
rest of the (v short) journey.

Got back into the car later and speedo did the same thing again - not go
backwards, increasing itself over time until it's at the 5 o'clock
position.  Rev counter works.  Tried turning engine on and off again a
few times - no improvement.  This morning came back to it and it's still
stuck at 5 o'clock.

Any advice?

I've had the speedo etc do weird things when the earth strap broke, but
that was replaced a year or so ago now and everything else works fine.

I know the speedo is electronically connected - and chance I can just
disconnect the battery and all will be fine?  Might I need to remove the
instrument cluster and take a look, or am I looking at a complete panel
replacement?  If so, any suggestions as to the sorts of prices I'm
looking at?

Thanks

David