The overbubbling is generaslly related to excessive heat... you may try a frosted bulb or a dimmer, to answer partially to that other question you posed. Anyway I believe that the column or the overbubbling depend on the quality and age of the wax.
By the way, I suggest that the column either is undesirable, the best would be a slow production of consisten bubbles mixed with that continuous pillar at times.
Yeah looks like overheating and possibly bad wax... the broken heat coil wouldn't help things either...
I've just started running all my lamps off dimmers and it's made a huge difference to the flow - heaps better! You can really control the flow plus you can run them longer too. Good luck!