Some regulator and OPD valve combinations aren't the best of friends. Try this:
1. Close the full cylinder via the triangular turn handle on the top.
2. Turn the regulator away from the full cylinder. (These first two steps are just to get you back to the starting point.)
3. Turn the regulator back to the full cylinder. Your indicator on the regulator should be reading red at this point.
4. Now the important part: VERY slowly open the valve on the top of the propane cylinder. Turn just a tiny bit at a time until open. You should see the regulator indicator switch to green; you may also be able to hear a swish noise as it happens.
If the regulator goes to green, then this was likely the problem. If not, the problem lies elsewhere.
Oh, just in case, you note that you're getting alternately "no gas" and "gas." How are you making that determination?