1992 Plymouth Voyager High Idle

About a month ago my van suddenly started going about 40 mph on its own, and once it was placed in park, I noticed the idle was high. After reading a few web sites, I looked at the TPS and found it was totally dead. The TPS was replaced, however, after a couple of days the high idle came back. Since then I have replaced the ECTS, the O2 sensor, and my speed sensor (which was giving a code).

The high idle will sometimes go away for a couple of days, but then comes back. However, the idle keeps getting higher, and must be around 6000 rpm now.

The IAC valve was replaced about a year and a half ago. I have also checked the MAP sensor, but it seems to be working properly. With the engine off it reads 4.67 V, with engine on it drops to 1.5V, and with throttle depressed it goes down to about 1.1V.

If there are any other suggestions as to the cause of this I'd appreciate it.