slight misfire

[M3144]

Just noticed today, car started to very slightly over rev whilst at
traffic lights, from about 850 revs normal to 1000 on idle.

Then after 30 secs the revs dropped back to normal but it started
to stightly misfire, and backfire.

This has only just occured, so can anyone help please

Many many Thanks

[peterguk V6 KWK]

Air leak?

Check all vac. hoses very carefully.