John:
Two thoughts immediately came to mind here:
First, since you had previously had a breaker tripping before replacing the one bad fixture, I'd look very closely for burned feed wiring. The old ballast may have cooked a connection or wire someplace...
Second, since now you have two fixtures acting up, possibly harmonics on the neutral. How many fixtures total on this one circuit? Are these electronic ballasts?
Do any of the ballasts feel excessively hot? Compare temperatures with the fixtures not having problems.
Hope some of this helps!
