1989 spider stalls out frequently

Hey guys,

My car is having problems stalling out. This is a recent development that keeps getting worse and developed a few weeks after installing a muffler (don't know if that's related).

Whenever I'm not accelerating, the car stalls out. So if I'm slowing down, it stalls out. If I cruise in neutral, it stalls out, and when I come to a stop, it stalls out, regardless of whether my engine is hot or cold.

When this problem first started, it wasn't as bad, but now it's basically guaranteed that under any of those conditions, it will stall out.

At first I would have some time (about 10 seconds) that I could stop without it stalling out. In those 10 seconds or so, my Alfa would vibrate an abnormal amount before finally stalling out. When it stalls out, the battery light goes on but the voltage-o-meter (not sure what it's called) isn't low.

I did have an air intake issue (my car wouldn't start) a few months ago, so I refitted / tightened the valve (which worked) but didn't replace it (not sure if that's relevant, just trying to provide as much info as possible).

All help is very much appreciated. Thanks!