From my own experiences with Fluorescent fittings and I have installed hundereds of them, it sounds to me like more of the first problem, just the burn-in period, T8's take a while to get seated when first installed, with either controls on them.
However, if you do come across a flashing effect from a certain tube with magnetic ballast, I'd say replace the starter.
Failing that, replace the tube, dud tubes are not un-common.
If a tube stay's lighted at both ends after replacement, your ballast is faulty.
