1998 Ford Mustang Stalls Out

Lately my 1998 Ford Mustang has been stalling on me. I recently replaced the battery, so I know that's fine. It mostly happens when it's hot outside (I live in AZ though, so it's pretty much hot year round, so that may or may not play a role in this). It usually doesn't happen unless I've been driving for 20 minutes or more either. Essentially what happens is that my car will stall when stopped. I have a 5-speed, and this happens when the car is in nuetral most of the time. However, it also has happened when I am making a sharp turn. In other words, sometimes when I have the steering wheel turned it will cause the RPM's to drop and it will stall out. This can happen when the car is moving or when you just crank the wheel all the way while stopped. The car starts right up again after it has stalled.

I also want to note that sometimes when I start my car the RPM's will jump all over the place. For example, the RPM's will drop down to about 500 RPM then the motore will feel like it's catching itself and it revs up to about 1500RPM (this is all while the car is in neutral and I don't have the gas pressed). It will do this 4 or 5 times then settle down to about 750RPM, which is about where it usually sits in neutral.

Any help and/or suggestions as to what this could be would be greatly appreciated. Thanks in advance for your help.