Are the powervalve servo's opening with the throttle opening?
You can move them a little with engine shut off and then listen /look to see if they close when you switch key to on position- this won't tell you if they are working at 1/4 throttle but will tell you they 'work' at least.


You stated it has slowly happend over a month, electrical normal is intermitant or just plain on or off, but and air leak or mixture change can be slow to take effect -and get worse.
As you said you have checked the carby is nice and clean and jetting wouldn't have changed or need changing if you haven't modded it in any way.
Air lines to the 'x' pipe and 'y' pipe clear and not cracked?
Carb getting enough fuel after 1/4 throttle , possible starving and shutting off?
When it shuts off on you one time check the fuel bowls with a long flat blade screw driver to see if its empty


Just from personal experience did you take out air screws and clean them when you cleaned carby/ rest them?
it will only take the tinniest bit of dirt etc to block them

When it stops is it hard to start?