dual fuel tank and only one wants to play

[tex_waco_usmc]

As the title states... 86 GMC with 87 tanks and drivetrain (350 TBI with 700R4). Drivers side tank and sending unit are brand new and work perfectly. Passenger side got a cleaning (as best I could) and new pump. Worked good at first except the sending unit reads right when it feels like it. Now it wont even idle for long when running on the passenger side tank. She always starts right back up but wont idle. If you are driving and have to let off to brake and/or coast she dies. This only happens when running on the passenger side tank.

So confused. Any help would be greatly appreciated.