I see, so it may very well be the cart, do your drivers (speakers) jump all over when the stylus is on the LP?
As for what issues I was talking about, I was responding your question if the isolation feet would help... they wont help either the light flickering or the Pop sounds.
I have been down this road with a few tables, and what I have found was it was a grounding issue. your issue could be something completely different but it sounds like something is not properly grounded.
I don't know the stock arm on the 550, as mine is not stock, but id say test out the interconnects between the arm and the amp, if your ground wire has a spade on it, cut it off and trim back a tiny amount of fresh wire.
The table would have nothing to do with it other than the grounding and the RCA's. The arm is isolated from any of the internal electronics. So you know 100% for sure its the arm, or cart configuration.
if its not your arm/cart its your amp. on your head amp, or phono stage, is there a gain setting or even a setting for resistance?