There are several things that need attention on the Novatac 120. First, using either deoxit, or some other contact cleaner (isopropal alcohol will do in a pinch), wet a Q-tip (or other cotton swab), and clean the ends of the spring (the removable one in the battery tube), and the areas in the tailcap and head where the spring makes contact.
At the same time, make sure that the little, thin, silver rings in both the head and tailcap is tight. It is easy to see. I has 4 tiny little cutouts at 90 degrees from each other. You have to take some tweezers (or something) and go down in there and apply pressure at two of those opposing points and turn counter clockwise to make sure that disc is tight. They are notorious for causing those weird types of issues in the 120.
You can also try stretching the spring a bit, to make sure it's making contact, but it's imporant to do it the right way. DON'T simply "pull" it apart. You want to "unroll" it a bit instead, that way it keeps the diameter roughly the same.
Next, clean the tailcap spring and area in the head where the positive battery contact mates. Be sure and hold the head with the light up, and only apply cleaner to the swab. You don't want any cleaner dripping down into the head.
Finally, when you reassemble the light, make sure that you have cleaned the ends of the battery tube and where it contacts in the head and tail cap AND screw them together fairly tightly.
If you have one of the programmable 120s, you might consider doing both a full reset, and then a battery reset (which sets it for either a rechargeable battery or a primary battery).
It sounds like a lot, but once you've done it and eliminated the problem, you shouldn't have to repeat those steps very often, if ever. Mine was wonky at first, but after doing all the above steps, it's worked great for many years. Good luck