1995 Eclipse GS vacuum problem.

My 95 Eclipse GS started acting up about a week ago. When the engine is cold it idles at about 1k until I put in gear. The idle drops to about 200 then back up to around 600 - 800. The brakes are hard and the intake sounds like it's not getting enough air. This happens for about a mile or so before it seems to straighten out.

I've done a compression test, cleaned the intake and throttle body as well as the IAT, MAP sensors and EGR valve and tested them. I have also replaced the cold air intake filter and the upstream O2 sensor. I've also checked all vacuum lines.

I just got it back from a diagnostic test and the only code it's giving is the downstream O2 sensor which shouldn't cause vacuum problems. It's only getting 11 lbs. vaccum which is of course not enough. Only thing I can think of is maybe the exhaust is plugged so taking it in tomorrow for a back pressure test, but other than that, I have no idea. :banghead:

Took it in for a exhaust backpressure test and there was some blockage, but not enough to cause this big of a vacuum issue. I checked the brake booster and no problem there, also have replaced the throttle body gasket and IAC valve with no result. New plugs and wireset, new cold air intake couplers, new hose clamps and still no change.