Car runs great but every once in a while it won't start. I will wait a few weeks then give it a try and it starts up fine, runs for weeks sometimes months then for no reason will not start.
I was told there might be a computer under the front seat.
Is that true?

 

U gotta give more info than that. Assuming you can't force the no start the next time it acts up quickly check for fuel and spark and if no spark or see the pattern it makes when it tries to spark. What do you need to do other than let it sit to Get it running

 

The "computer" under the seat, forget about that!
I'd start by looking at the FI relay, Crank Sensor, fuel pump, figuring the no starts you are experiencing...

 

there are no computers 'under the seat' on a 740/940 anyways, the ECU (fuel injection) computer is on the right side of the passenger foot well forward of the lower door hinge, while the ICU (ignition control unit) is behind the dashboard, above the steering column, on the firewall. but they are usually the last thing to fail, they are extremely robust and reliable.

work down the troubleshooting list here, paying attention to the items for LH 2.4 cars, (unless you have regina) https://www.volvoclub.org.uk/faq/Eng...eSymptoms.html

 

The SRS "computer" is under the front seat...

 

well, SRS shouldn't cause no-start or stall conditions.