hot tub shock problem( foams )

I Have a CLB6L Club series spa , it holds 300 gallons of water. This is a new spa that I have only had for 3 months. I have had to empty 3 times already do to the fact that I was unable to get the alkalinity down (started out at 490, never got lower than 360) ,when I added the chemicals to it needed (ph down , metal and scale , shock bromine , bromide floater) I had a very large bubble bath. After refilling my alkality was at 480 and my ph was at 9 to 10 Had to add ph down 3 tablespoons a day for 1 week just to get it down!!! finally got the readings to were they need to be. Needed to shock the tub( after using it for a week) and after adding 1/2 ounce oxidizer it began to foam after 20 minutes!! why is this happening . Please help I'm at a loss.
thanks
cstrait