Lava flows for 3-4 hours then freezes in the "up" position

[johnny]

hi, i have a lamp that is about 9 years old. it sat in my cool, dry basement for 4 years and i began using it again at work. after 4 hrs my lava forms 1 big bubble that goes from the bulb upward but doesn't move, break up into smaller pieces, anything. is that an overheating symptom? it looks like the attached picture of a lava lamp: no motion! when i look at the lava blob i can see the wax flowing within the bubble, so i know it's melted. but the bubble itself just sort of sits there in suspended animation, swaying very gently.
my work is about 73 degrees and the lamp sits near the heater vent. i can't move it though. would a dimmer switch help, or is my wax broken? thanks!

on a side note i'm getting a red wax/clear liquid 52 ouncer tomorrow, can't wait!

[johnny]

i will have to get the model from work. it's a common looking store-bought (USA) lamp. it's a 52 ounce, 40 watt bulb. has a silver/aluminum pinhole base, don't know if that helps?

[johnny]

that's the type. my condition seemed "unique" in that when everybody else has overheating issues it results in tiny bubbles or a flat lump at the bottom of the lamp.
The electrical outlet is next to the heat and i have a cramped office so relocation isn't really possible. i think i'll try a dimmer and see how it goes.
Thanks for the help Bohdan!