I've got three of the buggars and as phil-boys, they never miss a beat unless you try and stick some funny paint through it. Never ever had to shake the thing to get it to feed paint, and on the rare occasion it has stopped working I just press the blue reverse button and 99 times out of 100 that clears any jam.
I've heard talk that you can adjust a pot inside to speed up or slow down the feedrate, and can also be used to soften it's use against brittle paint, or harder to try and push odd-shape paint down, but to be honest every single one of mine is stock and it works well with all types of paint.
If it is used maybe someone has had a fiddle with that? Maybe someone else with more knowledge could confirm or deny if an adjusted pot could cause the problem?
In the meantime, try getting it to work off your marker, and just get it to feed into a pot, stopping and starting it by stopping the feed, and see if it works ok then. Also fresh batteries, make sure it is clean, all screws are tight, etc, etc...
