Any idea what causes this?

gas starvation. your tank is clogged up (the sock is probably rotted, dead leaves in the tank, junk, sludge, etc maybe even a dead mouse in it). or the gas line is clogged up, or both.

my gas tank when we got it off the duster, you could push your foot through the side...

you can test with a gas can/fuel line straight to the fuel pump, but i dont think you can test that going down the road too easily.

sometimes you can just use compressed air and blow the gas line out. but your tank ... you have to remove them to clean them out, then you can clean it up and paint the outside etc...but by the time you do all that a $150 brand new tank dont sound all that bad. the only problem a lot of people run into is the sending units tend to not work right on some of them. keep your old sending unit, gaskets etc from the old tank.