One thing I like about the Flex 3401 is that it is an easy DA to repair. Since you are no longer under a warranty I would fix it myself. I had a similar issue after beating the snot out of it. I have dropped my Flex on more than one occasion. Everything started to slow down and actually took a while to start going to the proper setting RPM. Then one day it just stopped working. I DO NOT have the extended cord just the stock cord it came with.
BEFORE YOU DO ANYTHING, check the gauge of the extension cord as that might be the culprit. I have heard if you do not use the proper gauge of extension cord, it will not function properly. I would ask Powerhouse if that is the case.
If that isn't check your carbon magnets (more than likely fine) and the trigger piece. The Flex 3401 is modular in design and parts can be easily replaced. I replaced my trigger and my Flex works great. I think that part is like $90 but that was almost 2 years ago. I hope I helped a little. THAT WAS MY ISSUE and may not be yours.
But based on your description if it isn't the gauge on the extension cord then replace the trigger module. If I had to guess....the extension cord is the problem.
But again a guess....PLEASE let us know the outcome so us Flex guys will have additional ammo for future repairs on the Flex
