93 GrandAm Idle & Oil Press drop w/ brake pressed

93 GrandAm SE with 65K. Just had the exhaust system replaced & mechanic says car won't start, it needs a new intake manifold. New Intake, complete tune up (sparks plugs, fuel filter, oil change) radiator and trans. systems flushed and serpentine belt replaced. I get the car back and it has more pick up but idle, rpm and oil pressure gauge are going insane. If driven at a constant 55mph the car runs fine with normal oil pressure of around 70 but in stop-n-go traffic it runs a ruf with oil press. dropping to 30. When @ a stop idle fluctuates jumping car forward as if pressing on gas while RPM goes from 1000 & drops to 500 rhythmically along with the headlights until I accelerate. If you attempt to stop again relatively soon idle takes a dump along with the oil pressure and the car jerks to a hault. If I restart immediately I can't exceed 30mph w/o it cutting back out but if I wait 15-30 minutes before starting I'll get a good 15 minutes of stop-n-go drive. Can anyone help me before I sink anymore money into this thing. ***Had multiple computer scans and no codes come up.:uhoh:

from what you are describing, it sounds like you are having a pressure issues. maybe your oil pressure is not staying consistant. If this happened right after you got it back, it's possible that the mechanic didn't put something back properly. I

You didnt say what engine but your IAC may be bad or needing cleaned.
And as grfnkl said, something could have been put back together wrong :eek:

Mech says it needs a new this, that and the other thing .Has any of it been done? How was it running before it went there ? I`d be cleaning Iac valve and port as well as throttle body ,before going to far ,also check it for vacuum leaks. get the idle straightened out and the fluctuating oil pressure will likely be ok. if you don`t really know this mech ,maybe get a second opinion from another tech before dumping lots of cash into it

ya, sounds like a major vacuume leak causing the idle prob. As the vehicle ages rubber and plastic parts become brittle and when handled tend to break easily. Also, when you reattach the rubber hoses they may not stay on as wel because they stretch a bit.

Sounds like your mechanic is taking you for a ride to me ... I've never heard of replacing an intake manifold ... unless he was just talking intake manifold gasket ...

How did the car run before you took it to him? definitly get a second opinion ... and if you feel comfortable with the second opinion show him the list of things that were supposably changed by the other guy & ask if they were done, & if the other guy could have caused these problems. Sounds to me that your car was running fine, you wanted the exhaust changed & then he told you that you needed alot of work, then you get the car back & it runs bad ... so now you keep coming back & getting things fixed (changed) until you get tired of making a car payment to him every month & go buy something new ... just my :2cents:

According to your title - idle and oil press drop with brake pressed - I can smell problem between your intake plenum and power booster, first check hose if it is tight on both sides and no leaks.
If you also have problems with braking ability I would check your check valve at the power booster, (plastic L shaped part that connects your upper intake manifold plenum to power booster who knows maybe he instaled your check valve the opposite way. Your symptoms also sound like bad or disconnected MAF sensor. So check your check valve - you should be able to suck air from the side where the hose is connected but you should not be able to blow air from the side that goes into your booster. If you have it the other way then that is your problem. Also booster can have bad vacuum diafragm (not sure if this is spelled correctly) but start your car in park press brake 3-5 times hard and if your rpm starts to act like you explained and if your brakes are worse now I would concentrate around booster.