'98 Pontiac shaking and service engine soon light blinking, car shutting off.

I recently bought a '98 Pontiac Grand Prix. It's given me trouble since I bought it. It has
over 200,000 miles on it. I realise now it was not a good idea, but I just needed a cheap
car for 6 months. It's been anything but cheap.

I've had to change the radiator, hoses, thermostat, water pump, engine filter (?), spark
plugs, EGR valve and it recently had a transmission change (allegedly, this is what the
arsehole who sold it to me told me). Now the 'check engine soon' light has started
blinking, the car is shaking at low speeds or when I'm stopped and as I was slowing down
today the car just shut off. I was able to start it again, thankfully. But I'm terrified to drive it.
Does anybody have any suggestions about what it might be and how much I am looking
at? I saw it could be misfiring.

I'm hesitant to take it to a mechanic without some idea because I've been screwed over so
many times by mechanics here. They see a young woman who doesn't know cars and
they know they can take advantage of me.