a reostat either starves the appliance for electricity or gives it too much. YOu might want to put a less wattage bulb in the socket and it might not flicker when full up.
I am assuming that the bulb might not be getting full power to operate properly and its like a automatic transmittion "hunting" when between gears and on the rpm dividing line, it cannot decide to go up or down in gears.
See if this works.
If leary about this, then remove it. I always like updated electrical devices in updated homes.
Your wiring is not alum is it? If it is, then do not use the reostate.