So I am at work right now and on my lunch break.

So I am at work right now and on my lunch break I went out to grab a smoke and sit in my car with the heat on. As soon as my engine started up, the RPMs started going crazy. My car revved up to over 2000 RPMs and was attempting to go higher. I turned it off completely and started it up again, this time it went to 2000 RPMs and was looping? Where it would go up and down around the 2000 RPM range up to about 2300 RPMs. I turned it off and tried again, this time like the first it just held at high RPMs. When I pressed the gas once, the RPMs went up as expected and attempted to maintain that RPM, about 2300.

Switching to any gear other than park sent the RPMs to a normal level around 1000 RPMs and under. However, when switched back to park from another gear, such as Drive, the engine starts revving up again to around 2000 RPMs

I am worried about driving home now after work, as an engine revving itself up scares me the death. This is the first time this has happened to me and I was wondering what may be causing this, and if I should drive home or have my car towed?

If it helps any, my car is an automatic transmission with a rebuilt transmission in it (placed in last year). I also recently had the rear O2 sensor replaced, as it was causing me problems as well. The transmission has already been having problems with getting into first gear, it does that very rough and jerks a bit nearly every time, though most of the other gears shift pretty smooth. I tried taking it to the shop that rebuilt the transmission and they wouldn't repair what was wrong with it without me paying full price, which I can't afford. I was told the transmission cable was slightly loose and that some of the "connectors"? Were slightly misaligned.