2003 Vauxhall Corsa speedo flips to 60 at standstill

My speedo goes to 60 and sometimes counts up on milage when I'm parked up and not moving. I thought it was only in the mornings as also seemed abit rough on starting (had to pump on gas to get it going) but I then realised it was when I turned on my main beam headlights. I have these really bright blue lights all wired up (I want these gone). If I turn them off speedo reading goes down. While the lights are on and I'm moving the speedo is fine but as soon as I pull up at lights or something it flips to 60 again.

This does not happen when the lights are off!

I didn't realise it was related to lights so took it to be looked at and the fault showed the 'spark plugs' behing the dashboard and replaced them he said it was also why it was rough in the mornings.

Turning over the car is now fine, but speedo unchanged? Any ideas? Apologies for the lack of technical wording but hope someone can help