I don't think it is electrical. Mine did that (oddly) when the pretty multicolored vacuum lines cracked. I found it went out when under full throttle which didn't make sense to me (low vacuum?!!) but I didn't care so I sealed the vac line and pulled the bulb. Probably didn't need to pull the bulb. Dunno. But it's not on anymore
but I have a homemade posilock cable. If you are using the vac operated disconnect this won't work for you.
There are two kinds of 4x4 light switches: one is a vac switch (like mine) and the other is switched at the vacuum motor by a mechanical switch that depresses when the motor moves. Either way I think the switches merely ground the powered bulb to illuminate it: I think, not sure. So it could be a chafed wire, a stuck plunger switch, or a vacuum switch issue.