Bump @ 3 and 9 during helical interpolation

Have an older Cinci Lancer 2000. When I am interpolating say a 5" Dia. hole for thread milling I notice the following. When the cutter gets to the 3 o'clock position the table will bump same at 9. Sometimes you can actually see it, most times you need to place your hand on the table to feel it. I have checked the dimensions of hole with the best metrology equipment we have and it's fine (I realize wax an't gonna be great for holding any kind of dimensional stability). What bothers me is that when I go to thread mill this might pose a problem. I have a couple ideas of what might be happening but would like some expert opinions. Never thread milled on this machine before so I got a chunk of wax to do dry run on. I am running a 1" carbide em @ 300 rpm feeding at 10 ipm with 10% cutter engagement.

Thanks,
Doug