'98 Vectra speedo problem

Intermittently, the speedo and rev counter are not working- both show 0, but
the speedo needle vibrates up and down a bit, keeps on doing it for several
seconds after switching ignition off, then sometimes jumps up to about 5mph.
Come back to the car and start up after 1/2 an hour or so, all works
normally. All other instruments (temp, fuel, warning lights etc) seem to
operate correctly when the speedo isn't.

I'm guessing it's the speedo unit itself I'll need to change, but I'd be
very grateful to hear of any other suggestions.

 

Yes, was the speedo, got another from a breakers and it works correctly.
The one I got must have been from a larger engine car, max 160 mph, mine was
max 140 mph, but it seems to show correct speed.

 

Well, for a start, if the needle goes to the same position (angle), then it
will read higher than it should, but from my experience of driving on roads
I know well at speeds determined by other drivers, and by feel, I get the
same indicated speed.

Anyway, speed policy is becoming so stupid that unless you want to fall
asleep at the wheel, you will probably be driving at what feels a safe
speed, and concentrating on detecting scameras and braking for them, as well
as looking out for real hazards. I can no longer drive at many recently
lowered limits, just have to do my best to avoid being detected. Because of
this, it's becoming less important what the speedo reads, I just have to get
the car through the mot.

If we had sensible speed limits sensibly enforced, I'd be more concerned
about it. But then, the scamera partnerships wouldn't make the millions they
make, so I'm sure it's a downward spiral.