Pontiac Repair: g.m.1998 grand prix, high octane fuel, mother inlaw

Question
My mother in law has a 98 grand prix.  He car sometimes shakes after start up. when this happens the engine light blinks.  the car will ususally calm down after the car warms up. sometimes it will shake utill turned off and trhe engine light stays lite.  other days the car is fine.  she tells me that at a red light the car is at the verge of stalling.  I noticed when it happend when i started the car in park that as I accelerated the car in park up to 1000 RPm it shook the worst. then if I took my foot off the accelerator the RPm droped to around 500 or so then leveled off to about 700.  if I accelerated to 2500 RPm then took my foot off the accelerator the rpm dropped smothly down to 700 not going below 700.

My mother inlaw has had the car to the dealership they have no clue.  they hooked up a recorder box and A trigger buttton in the car. My mother inlaw is supposed to pres the red button when the car shakes. the dealer was baffled because the car did not shake when she took it to the dealership.


I had put injector cleaner in her gas and told her to use a high octain gas. the car ran smooth for  a month,  but now the shaking has returned.

The dealer had suggested trying to replace all the injectors at a cost of $300.00 each I feel they are just guessing. they have alreaady tried to clean all of the injectors and a week later the car was shaking again

please help to shed some light on this problem. this is her second grand prix her first was an 89 and a lemonand, this one looks to be the same. I own a an 89 honda and my car has never had the problems her granmd prix has.  I hope you can help.

thanking you in advance for all your help.  

Answer
Well the most common cause of her problem would be clogged injectors, but if the dealer cleaned them correctly the problem should have been fixed. She should NOT use high octane fuel. Use the 87 octane that the car was meant to run on. It would help if you would have told me what engine she has in her Grand Prix because different motors have different problems. If she has a 3.1 or a 3.4, she could have a corroded coil or a plug wire. If she has a 3.8, she could have a slight coolant leak into the intake manifold.
The dealer should have been able to retrieve a code from the computer to see which cylinder or cylinders are misfiring. They should know the things that I just told you about and know how to repair them. If there is another dealer close to you, you might want to try taking it to them and see what they have to say about the problem. It should not be that hard to find her cold start misfire condition.
If you need anything else, feel free to ask.