Help diagnose my trouble codes

My car is a 2002 Toyota Highlander V6 3.0L (1MZ-FE). 111K miles on it. Averages about 16-18 MPG which I think is lower than it should be. I only bought it a couple months ago.

Problem is the engine stumbles while holding the accelerator steady and cruising between 50-55 MPH. If I let up, it recovers fine. If I put the pedal to the floor, it accelerates normally. I haven't nailed this down exactly, but it seems to me this problem is more prevalent when it's wet or raining out.

The Check Engine Light came on shortly after I bought the car, tonight I finally got around to borrowing a code scanner from the local auto parts store to check the codes and reset the light. Here's what I found (in order):

P0171 - system too lean
P0174 - system too lean
P0136 - O2 sensor circuit Bank 1 sensor 2
P0300 - random misfire detected
P0301 - cylinder 1 misfire detected
P0301 - cylinder 3 misfire detected
P0305 - cylinder 5 misfire detected
P0171 - system too lean
P0174 - system too lean
P0136 - O2 sensor circuit Bank 1 sensor 2

I came home, started googling, found this site, read a bit about air leaks and MAF sensors, so I lifted the hood, pulled the MAF sensor and found it nice and clean so I put it back. I did a quick check of the air hoses and found nothing obvious, but I wasn't exactly thorough - it was already dark out and I only had a little headlamp on.

After reading some about O2 sensors and their effects on how the system regulates the fuel mixture, I'm wondering if a faulty O2 sensor could be effecting the fuel mixture, making it run lean and causing the stumbling problem I'm having. After looking at prices for O2 sensors, I'm hoping I can get some help troubleshooting before I start throwing parts ($) at this issue.

Anyone got any bright ideas about how to troubleshoot further so I can narrow this down a bit? I troubleshoot for a living but it's computer networks, not engines.

System lean on both banks would ,ake me think there is a vacuum leak. Are you sure the tiny little wires in the MAF sensor where clean.

It looked like this right
http://partimages.genpt.com/partimages/296087.jpg

Yep - that's the part. I could look down the big tube and see two itty-bitty wires in there and even see a little bit of the PC board they're connected to. Everything was nice and clean.

So do you think the O2 sensor codes and the system lean codes are most likely unrelated?

I don't think so. The 02 code appears to be for a sensor after the cat. To be honest with you if I saw that many codes on a vehicle I was working on I would clear them and see what comes back and then focus on the returning codes. It is very possible for those codes to be in memory for a very long time if they have never been cleared. Starting fresh might be a good start.