Consistent light/dark cycles are what this stuff is all about. It will stress any plant, going into and out of flower.

1) Did you cut-off the 'ground' on the 3 prong plug, so it will fit the two prong timer? (can fry both timer, and ballast, but is a good way to check your fire sprinkler system)
2) Is the bulb securely screwed into the base? Recommend checking (while lamp has been off for a while) every once in a while.
3) Using a GFI?
4) Are you sure the timer is not just 'skipping' the red 'off' insert? I always use two red ones back-to-back on the dial, for just that reason. This works fine, on my cheap Intermatics timers.
5) And finally...do you keep buying the same timer, expecting different results?
Rusty Trichome Reviewed by Rusty Trichome on . A night of light. Quick Qs Hey, Being a student, I'm out of the house all the time, and my 400w HPS keeps busting the balls out of any timer I plug it into after about 4 days. It's not cool. I have bought three so far, all rated up to 3200w. Anyway, this isn't a rant about timers, it's an honest question. As often as I can, the lights go on at 11pm and go off at 10am, 13hrs dark to kick flowering off. But, for many reasons, I'm often not in my house at those times. Just today, my light spent about 16hrs on, Rating: 5