2002 Montero Sport EGR Codes

[TowBar]

I have a 2002 Montero Sport. The Service Engine Soon light came on. I read the code, and it was P0403 (EGR circuit malfunction). I replaced the EGR valve solenoid, which seemed to correct the problem. A couple of weeks later, on a rainy morning, I drove through a huge pond of water on my way to work. Just before getting to the parking lot at work, I noticed the engine idled rough when I came to stops. When I left work to go home, I noticed the rough idle problem still there. On the way home, the Service Engine Soon light came on again. I read the code and this time it was P0401 (EGR insufficient flow). Doing some quick troubleshooting I discovered the EGR valve would open as soon as the ignition was turned on. This is what was causing the rough idle. On a whim I re-installed the old EGR valve solenoid that I had previously replaced. This fixed the rough idle problem, but did not clear the P0401 code. I removed the EGR valve, checked for carbon build-up, and found none. I ended up replacing the EGR valve, and resetting the computer to clear the code. This seemed to fix everything for a few days, then the Service Engine Soon light came on again. Now I am back to the P0403 code. I could try buying another EGR valve solenoid and replacing that again, since that cleared the P0403 code the first time, but I hate to keep spending money by guessing what the problem is. Before I replaced the EGR valve solenoid the first time I had measured the resistance on the power connector port on the valve, and it read less than a couple of ohms of what it should. I didn't think that was a big deal, but replaced it anyway. After having the rough idle problem, I measured the resistance on the power connected port of the new EGR valve solenoid that I had installed, and it was way off. That would indicate that it had crapped out. Any other ideas rather than replacing the EGR valve solenoid again? I am seriously starting to doubt that it is bad.