What is wrong with my ford crown victoria?

I have an 02 Police interceptor crown victoria, it has 130,xxx miles, I bought it 2-3 months ago, and didn't have a problem. Out of no where my check engine light came on and stayed on for about a week then went off. Now the past few weeks every once in a while my car will rumble and jolt a little at stop lights, and when driving from a stop it jolts back and forth like I'm hitting the brakes, once I get to about 45 mph it drives fine. It doesn't always jolt around, just when it feels like it, I haven't been able to afford getting it checked out, but does anyone know what this may be caused by? I was thinking it may be a vacume hose, because my old car had a similar problem and it ended up just being a worn out vacume hose, any ideas as to what is wrong?