Timing/idleing issues

[240dk]

Ok so I have been trying to set the timing on my sr, I have already pulled the CAS and realigned it. My problem is tryin to set the timing, Im supposed to set it at 800rpm, but I cant get my car to idle at 800rpm!! When I turn the idle screw down, when the rpms get below 1000 it revs up and down, until it finally cuts off. It goes from like 700 to 1100, then back down and back up, so i cant set my timing!! anybody know whats going on here? I have checked for vac and boost leaks and cant find any, also if I hold the rpm at say like 5000, it goes up to 5200, then back down, and the vac gauge flucuates with the rpms. Please Help!!!