I have had a spa (now on my 2nd) for about 12 years. Usually, I have found that foaming comes from body lotions, oils,fabric softener etc. I have tried all the anti foaming agents around, but once you get foam the best way to eliminate it is to drain the spa. I have not experineced the "poorly rinsed" bathing suit, which was mentioned, but my wife rarely uses fabric softener, so that may be the difference. If you use lots of lotions and body oils, take a shower first!
Good Luck!