As a long time NovaTac owner (from the original release), I can tell you that keeping ALL the electrical contact points clean and secure is paramount for proper functioning of the light. I kept having intermittent issues with my 120P for months until I finally figured out that it simply needed a good cleaning and "tightening".
First, be sure that both of the slim sliver retaining rings are tight. There is one is in the head, and one in the tail cap. You can use needle nose pliers or tweezers to grip the small indents across from each other and twist clockwise to tighten. Next use some contact cleaner on a Q-tip and carefully clean the battery contact points in the head and in the tail cap. When you clean the head, hold it up so that no cleaner drips down inside the electronics. Also, make sure you don't leave any Q-tip fuzz behind. Might as well clean the contact points on the battery while you're at it.
Next, make sure your spring is long enough. Don't pull it, as that will make it narrower, but rather "unwind" it just a tiny bit, then clean both ends of the spring. Now, using a clean Q-tip and contact cleaner, clean the areas in the head and the tail where the spring makes contact. Finally, clean both ends of the battery tube.
Next, secure the battery tube to the head and make sure it is secured tightly. Finally, Insert your battery with the spring and secure the tail cap, making sure it's tight as well.
I was having all kinds of weird issues, and this simple procedure cured them all, and with periodic maintenance has kept the light running great all these years. Good luck!
