1997 buick lesabre series 2 intermittent crank no start.

So last year I bought a 1997 buick lesabre custom with about 76,000 miles. Drove it home, about 500 miles no problem, I had the freon filled because it was low. Shorty afterwards, it starting cutting off. So I had the alternator changed, drove it for a couple miles and stopped at o reilly s for a couple minutes. Came outside and it was cranking but it wouldn't start. We changed the fuel pump and still couldn't get it to start. I literally left the car there for about a week out of frustration. I came back and it started right up. An electronic mechanic told me it was the alternator harness . So he rewired it to stay on but the problem persist. This happens when the car is at operating temp. Also I live in south texas so during the high heat/summer is when the problem starts. It runs good when I'm on the highway ,but when Im driving in the city/stop and go or idling at heavy traffic red lights it will turn off. Please help, running out options. I bought this car thinking this motor the 3800's were fairly simple to fix but I was mistaken. 

Jzap574 wrote:

It runs good when I'm on the highway ,but when Im driving in the city/stop and go or idling at heavy traffic red lights it will turn off.

 

My first thought is a dirty throttle-body. If it gets too dirty, it will not allow enough air past the throttle plate, at idle.

i had one do this a couple weeks ago, it ended up being the crank sensor. Intermittently wouldnt crank after getting hot, let it cool off and it would crank again.  i had to use a labscope to verify i wasnt getting one of the 2 crank signals to the Ignition Control Module. Replaced the crank sensor and it was fixed.

Appreciate the advice, ill take look thank you.