Idle issues/queries (new owner)

[Mb426]

I’ve just bought a 1999 Impreza turbo and recently I’m having some issues with idling. On cold start it seems to be somewhat fine however on a new tank of petrol I’ve noticed either misfire issues or some sort of idle problem. The car when I clutch in shoots up to a steady 3k RPM and idle it stands from 2.5 to 1k rpm. Just looking for a little bit of help I’m hoping it’s maybe a spark plug that is shot. Could it be a sensor issue a compression problem or anything else any tips or guidance would be greatly appreciated as I’m a new member thanks for your time.

[ROSSCOSM]

Is the car moddified at all? And did you put super unleaded in? 3k is fairly high not sure how the classic differs but im sure my 04 idles at 1500 ish then slowly creeps down as it heats up. Could be a few things possible someone else has had similar issues

[Mb426]

No modifications at all other than an exhaust including the manifold not sure if that would make a difference? And when I filled it I put in just the standard petrol that goes in and I believe it’s 95 octane.