I'm not sure how much help you are going to get here. We do things more on the emulation end.
My Halloween obsession gives me a *little* experience with compressed air systems though.
Ok the left valve is failing so it's something on that set of lines. Fill the whole system with air, then take all the lines running to and away from that value and coat them with a little soapy water, especially the joints. Now run the test.... if you see any small soap bubbles anywhere, you've got a leak.
Otherwise, it actually is the valve as reported. Clean all the electrical connections going to that valve, use a multi-meter to test the voltages as indicated in the service manual and if it still doesn't work trying replacing the valve. If that doesn't work then it could be an issue with the drive board and...well good luck with that.
That resistance you mentioned could be the issue as well. I'm guessing it's just a symptom of having low air on that side, but if the game uses air-powered pistons, it could be that the left one is mangled, which would cause air to leak out of it and keep it from moving smoothly.
That's just all generic advice.... I don't know hardly anything about the physical machine itself so it's hard to troubleshoot blind.