Stalling 2003 Pontiac Aztek

Suddenly, my car stalled out on me about 15 times going for 3 miles. My mechanic told me that the car thought it was being stolen and identified that I needed a new computer to replace one of ten in car. About 2 weeks before the stalling event, I had changed the car fob from the 15-year-old plastic cover to a new cover from Amazon where you just put the chip inside. My mechanic did the replacement of FOB but he doesn’t think that has anything to do with stalling. Should I consult a dealership-- in order to determine if this is the ‘end’ for me and my Aztek? (The mechanic has identified a used computer, but says he can’t get new GM one as they are backordered due to age and make of car.
Advice please! Go to dealer? Possible that FOB is the culprit? THen, if it is, how to donate/sell now unreliable Aztek… She has over 91,000 miles, but she has been a total peach.

Your mechanic is wrong

The passlock system will prevent the engine from starting, if a valid key is not recognized

It will not shut the engine down, once it’s running

it’s probably something else, for example a crankshaft position sensor or a fuel pump

Thanks so much db4690! Do you have a view as to (1) whether the car can be better serviced at a dealership or stay with my mechanic who has cared for this car since 2008 or 2009, and (2) since I am not a handy person with cars is it time for me to consider donating the car to a worthy place? (I don’t think the car has much ‘value’ in $ - though she has served me very well!)

Just from your first post I really don’t have a lot of confidence in your current mechanic. A dealer might be able to find the problem but they will not do it for free. They will also have a list of things to do and it will be up to you what you will have done.
As for donation , don’t expect much of a tax deduction so I would sell as is or scrap it and donate the cash instead.

Thanks, Volvo_V70. Appreciate your perspective on both my questions.