[denverpete]

Loose connections are always a possibility, particularly if any of the wires are tapped.

If none of those suggestions work: It could be a faulty ignitor, the ballast assembly was placed too far away from the lamps, incorrectly wired ballast/ignitoror, incorrect (reversed) wiring to the lamp, or the lamps are failing. Each of these tends to produce the flickering you are talking about.