2002 Chrysler Town & Country Question: 2002 Town & Country. My engine speed speed is fluctuating and idling fast.

I have a 2002 T&C, 3.8L engine. At first, it was stalling at low speeds or when idling, so I replaced the throttle position sensor. That stopped the stalling. Then, it began idling fast (1500-2000 rpm) and shifting hard in low gear, so I replaced the idle air control valve. That helped a little, but it still idles high (1100-1200 rpm) and engine speed now fluctuates when driving on the highway. I got a engine code p0508, but that went away when I replaced the IAC. I also occasionally get the p0455 (evaporative system leak). I have replaced the vacuum pump and had the lines smoked, but they can't find any vacuum leak, so I don't know if that is related. Any ideas?