Okay well on saturday I decided to go to formby beach (like every other person in a 20 mile radius) and I was concerned about my coolant as my car tends to overheat sometimes.

However whenever I got stuck in traffic the revs on my car (mk1 55sx) started bouncing between 1500 and 2000 rpm then calmed down to below 1000 then slowly rise back up again. I assume this is the idle control valve? If so how much are they, and are they easy to replace?

I also just fitted my own induction kit a few days before, not sure if this has anything to do with it, it's like a GSR just home made

Drove my car all day today, seemed okay although there was no stop start traffic. Still anyone know?

Most likely induction. If it does it again remove the induction and see if it stops