40 watt single light fluorescent fixture problem

I replaced the kaput magnetic ballast in a 40 watt single light tube Fluorescent fixture with an electronic ballast made by Keystone (Fluorescent tube F40T12 with a Keystone KTEB-14-1-TP-EMI). Now the Fluorescent tube lights up on each end but not much in the middle. Several years ago, I ran a separate bare copper ground wire to the fixture and connected it to the grounding screw on the chassis to ground it and made sure it and the ballast are grounded. Tube sockets on both ends are almost new. Original wiring the white and black wires from the ballast are connected to is old so may be corroded a bit. Replaced the single pole wall switch with one that is almost new. If I turn the fixture on and wait a while, the Fluorescent light tube becomes as bright as day from one end to the other. Most of the time, though it's not on for that long at a time. Any idea what could be going on?