light flickering

An educated guess. I'll assume you are speaking of a neon test light.

In a neon device, the anode and cathode develop voltaic differences within a neon gas field which allows current to pass causing the neon to emit light energy. In this process, the resistance of the neon gas increases slightly making the unlit neon a lessor path of resistance, causing the electrons to change path, resulting in a momentary reduction in light output until the new path can be fully established.

Thats my guess.

Tim Gard

Since in an arc discharge the v-i characteristic exhibits a negative resistance characteristic, the resistance of the gas decreases as the current increases (hence the usual need for some external current limiting). The flicker is more likely to be that at the end of each half cycle, the current goes to 0 and the arc goes out. It will not re-establish until the voltage has risen to a certain level so that 120 times a second, there is a short "dark" period. The same effect occurs in fluorescent lamps but the phosphors on the tube smooth this out so less flicker is noticed. A bare mercury vapour tube will show much more flicker than a phosphor coated tube (and has less visible light output).