How to fix the gas pedal moving by itself

I bought this car knowing it was a fixer-upper and needed a lot of work. Eventually I got most everything working and finally was able to drive it without the check engine light constantly lit.

However, one problem has eluded me to three years now, and I just finally solved it!

The symptoms were that while cruising at a constant speed, the gas pedal would suddenly start to move under my foot! It would accelerate and decelerate, usually only within a range of 3-5 mph, and if that was all it was I might have been able to ignore it. But every once in a while, it would basically stall out the engine; it would still be running, but pressing on the gas didn't do anything. While the pedal movement could start happening at pretty much any speed, it only ever died when I was on the freeway doing about 60 to 70 mph. I eventually got to the point where I could pop the car into neutral, turn the key off, restart the car and put it back in drive, all within about five seconds without stopping the car. Because restarting the car always put it back on track and stopped the pedal movement - no guarantee it wouldn't start up again, though, but since it didn't happen every time, that was what I did at the time.

I checked the scan codes with an OBD-II reader, and there were never any trouble codes or pending codes set, but I could see that there was freeze frame data available; unfortunately AutoXRay wanted a hefty sum to unlock that feature on my scanner, so I never was able to use it.

I've put two MAF sensors into the car, replaced all the oxygen sensors, the catalytic converter, pulled the throttle body and cleaned it, checked for vacuum leaks and could never find the problem. Research on the internet didn't help either, so I lived with the problem for three years.

However, I ran across a listing on RepairPal that talked about a similar problem, which turns out to have been caused by a broken vacuum hose to the pressure regulator

Wanted to make sure I knew which hose they were talking about, so I did another internet search and came across this page:
I followed their steps and found that the hose to the pressure regulator was fine. However, when I tried to pull vacuum on the hose going to the EVAP pump, I couldn't get it above 10 inches of mercury no matter how fast I pumped my little Mighty Vac. And the pressure would fairly rapidly drop back to zero. (The hose going to the purge regulator valve was fine too.)

AHA! Definitely a problem here! So I jacked up the car, pulled off the right rear wheel well liner and located the EVAP canister with the pump mounted on it. There was a rubber hose between the pump itself and the long nylon-like hose going to the front of the car, so I disconnected it and checked for vacuum from the rubber hose to the pump itself. It went up to 20 inches and held! So, the problem must be in the long lines going to the front of the car, right?

I plugged the end of the nylon line and went back to the front of the car to try pulling vacuum again. To my surprise, I could pull 20 inches and it held! Wtf? Okay, I reconnect the hose at the back of the car the way it was originally and try it again. *It's still holding vacuum!* Only thing I can figure is that the short rubber hose at the pump had come unsealed, and the process of pulling it off and replacing it seems to have resealed it.

So I put everything back together again and retested one last time, and it looks like everything is good; 20 inches of vacuum and holding on all three lines.

And, of course, the whole point of this being that I was trying to fix a surging gas pedal, so I took it out on the freeway, accelerating hard, cruising along and varying my speed to try and get it to start misbehaving. Not even a millimeter of movement from the pedal now.

It's been three days now and it seems to be fixed. Weird that the EVAP system could screw up the throttle so strangely, but that seems to be the case. I'll post an addendum if the problem comes back, but at this time it looks like problem solved - FINALLY!