You can eliminate your airbox thermostat theory by pulling the silver tubing from the exhaust manifold thereby eliminating the hot air intake. Bucking at 40 mph,clogged fuel filter,clogged catalytic converter and or exhaust,clogged airfilter aka engine cannot breathe.If going up hills when it happens on less than a 1/4 tank I would suspect the in-tank pre-pump or filter sock or rotted rubber pickup hose.