The "Valve Gear" generally refers to the side rods and all the other associated linkages that steam engines have. Your 2-6-4 version will have simplified valve gear as compared to the earlier 2-6-2 version.
Sounds more like you might have a problem with a dirty E-unit. Spraying it with some CRC QD Electronic Cleaner (Walmart, automotive parts stores, hardware stores) should do a good job of cleaning it. Unlike the rest of the mechanism, the E-unit does NOT need to be lubricated. It needs to be clean and dry in order to operate properly. Also check for a broken wire or solder joint. This can easily be re-soldered if you know how.
The reversing lever is actually nothing more than an on-off switch for the E-unit. If on, it allows the Forward-Neutral-Reverse-Neutral sequencing to operate full time. If off, it simply kills the power to the E-unit. The contact drum will stay in whatever position it was in last, so the loco can only do one of three things - either go forward all the time, or sit in neutral all the time, or go in reverse all the time. To resume normal F-N-R-N operation, simply flip the reversing lever back to the "on" position, and the E-unit will resume working.
Also, visually check the gears on the drive wheels and motor shaft to make sure you haven't picked up a piece of dirt or grime in the teeth, which could be causing intermittent stalling. Clean up any dirt, then re-lubricate the mechanism. A light oil for everything that turns and spins, like axles, bushings, bearings, and even the valve gear, and a light amount of grease on the gear teeth. Go easy on the lubrication, a little goes a long ways.
You could run the mechanism without the shell in order to better observe it. If something still isn't working properly, you should be able to figure it out a lot easier if you can actually see what it is doing, or not doing, as the case may be.