surges

I have a 1992 Pontiac Firebird Trans Am. I is ok when I am driving it as far as I know, but when it is idels. It engiine will surge and the RPM will jump between 700 to 1200. then drop back down and smooth out. Then it will do it again but it never stops surging. After about 10 minutes it will die. I have tried a lot of things but nothing seem to works. I found a ground wire that that was broke and fixed it and the timing was off a little so I fixed that, but it still does it. HELP!!!!!!!!!!!!! any help will be appreciated. Thank you