My first thought is they pinched some kind of vent line for the EVAP system and therefore it is vac locking the tank.
But the irregular pressure differential I would think would set a check light.
Does it die while idling? And have you run it with the cap off.
Tp sensors can do the same thing as well.
Mine went bad on my 96 and showed itself on a rainy day with the moisture...no idle.
