Differing Tachometer Readings

Okay guys, everytime I start my car I have a different RPM reading on my Tachometer, which I believe alters my cars performance, but im not sure.

*Background info. I haven 03 Toyota Celica that im using as a tuner car. It has an after market intake, and exhaust. Also, the Catalytic converter broke a few months ago(around the same time I notcied my rpms changing). Also, its an automatic.

So, heres an example of whats going on with my car. Last Night, I started my car. It sounds boisterous and is idling about 1500 rpms. It ran smooth while I was driving, accelerated great and the exhaust made a sastifying rumble.

Now, this morning, both times I started it, the engine sounds weak, varying between 500-1100 rpm, and when in drive runs as low as 300 rpm. The exhaust sounds like its going to rattle off, and the engine makes a whining sound when im accelerating and feels like its stuggling.

Id just like to know what you guys think is wrong, and what I can do to fix it.

Im about average speed when it comes to understanding how cars work, but im pretty new at the import scene, so im completely lost as to what could be wrong with my car. I hope this is enough information for you guys, if not let me know.

Thank you.