odd speedo moment

Washed the car this afternoon, turned it on to take it out for a quick drive to dry the brakes off, initially the car over-reved but blipped the throttle and it settled back down. Out on the road and after 400 yds the speedo (not revs) starts dropping to 0 then back up then down again. Did this a few times, but no affect on the driving. Out onto the main road and gave it some beans and all settled down.

Any ideas?

Cheers

Mine had a similar moment…started it up reved really high…on the dial…engine was less the 1000 actually… then when i rev’d it was as if the needle was back t front…going down insted of up lol