Ignition wouldn’t change by itself, no, but it sounds like the car has never really been right.
The obd1 code scanners are $20 on amazon, a wise investment for the efi mustangs.
If the car is stock, then it shouldn’t be too bad to diagnose. Check basic things: such as fuel pressure mentioned above. Performing a base idle reset is also good to do on a car you have recently purchased.
If the car runs well when pedaling, the idle air control (iac) valve may be starting to go. If you unplug it and the idle doesn’t do anything, it may be faulty. They’re like $50, the bmd ones are still made in the USA.