Speedo needle flying around at 90km/h+

Hi,

My car always seems to get these weird problems people have never heard before or can't explain. Hopefully this time someone knows what's wrong and it's an easy fix.

So driving along the freeway today, on the on ramp, getting up to 100kmh and then I'm looking at my speedo and it suddenly drops to 0, then instantly shoots back to 100, 2 seconds later, exactly the same thing, and every 2 seconds from then on, it would do it, completely in beat, not erratic, not off time, every 2 seconds.
First thing I did was sh*t bricks, I hate it when my car does weird things at high speeds (like my steering starts shuddering and going nuts lol), anyway I tried to diagnose the issue and I have a theory.
First, I slowed down to about 80/85kmh, needle stops jumping around, reads accurately.
This led me to believe it was the transmission going into 4th gear being the issue, so I locked it into 3rd and got up to 95kmh, speedo starts jumping around again. So then I got it up to 100kmh and let go of the accelerator, and it stopped jumping. Just sat at 100kmh (GPS verified as correct speed).

So here's the issue: any speed over 90kmh, gear and overdrive independent, the speedo 'pulses' between my actual speed, and 0. Trip comp digital read out confirms 0kmh then real speed, and it's consistently jumping every 2 seconds (roughly).
If my foot is off the accelerator, the needle doesn't drop to 0, and any speed below 90kmh reads correctly.

I'm thinking speed sensor? The issue is my average fuel is lying because its moving the speedo around heaps, and the car won't go into overdrive because the speed is lying. It's as if the speed sensor pulses on/off after 90kmh

Any help greatly appreciated , thanks!

 

Ha you're a legend! It worked!
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