90 Lumina 3.1-Sporadic engine miss & check engine light

She is a one owner, well maintained, garaged car with 144k miles. I really want to keep her, and NEED input in diagnosing this evasive problem.
In the past 2 months, she missed & coughed 3 times, and the check engine light came on. Each time different circumstances, yet, all after being driven then sitting 1 hour.
One time on freeway going up a hill, the others on surface streets with no load on engine, engine even died during last bout, but started again instantly. All 3 times, I turned off AC, revved engine, shifted gears, and that seems to have temporarily corrected it, and the check engine light went out.
My mechanic did tune up and replaced spark plug wires, symptoms returned in 2 weeks.
Took her to agency for diagnosis and get engine codes. They said 90 Lumina has computer, but has no ability to store engine codes, and they could not diagnose the sporadic problem.
She is running purrfectly again, yet I am now hessitant to drive on freeway.
I would greatly appreciate any helpful ideas.
Thank you, Mana

your car will/should store the code for 50 key on/key off cycles. It sounds like a classic Crank Position Sensor going bad to me. If they didn't actually hook your car up, they were blowing you off. If they hooked it up and no codes were shown, then you may have an ECM going bad.

Dear RICHTAZZ.....I thank you so very much for your reply and expertise.
YES, it was the Crank Position Sensor. It was badly cracked all over, and sending sporadic signals.
I am so grateful that I now get to keep my beautiful, dependable Lumina, which I prefer over any 2006 clone.
Thank you again, Mana

You are very welcome, I'm glad I could help.