twin cam revving

Hi, I am new to this site and I am in desperate need of help. I have a twin cam corolla 1987.
It has been revving (all by itself), mostly it is when I have stopped and it just sits there and revs up and down. I have had some work done on the head but it still keeps doing it.
I think it has to be a sensor somewhere that is playing up but I am at my wits end.
Has anyone else had this problem or know how to fix it?
Thanks!!

It sounds like a leak in the vacuum system, tubing leak, etc...which are tricky to locate. Good luck.