Mold in Bathroom Whirlpool Tubs

Although my housekeeper keeps the house looking good (including my bathrooms and bathtubs), when I turn on the whirlpool jets in my whirlpool tub, small chunks of mold get blown into the tub. Apparently, the lines have mildew/mold in them.

How common is this problem and what can be done to correct it?

Instructions for cleaning “the works” of a whirlpool:

http://www.whirlpoolsystems.com/catalog/whirlpool.htm

I guess it makes sense that any sitting water would mold/mildew in the lines over time (esp in warm weather) but it wasn’t the first thing I thought of re: whirlpool tubs. It will be now,

Jois