Could be just bad luck... could be a bad capacitor on the control board (I'm doing a bit of wild guessing here).
Because of the high resistance your getting the switch is "throwing" properly, there is just something on the switch contacts raising the resistance from what it should be to (near to zero) to what you're getting. Now... I *think* the type of switch you use is a magnetic reed type. It's a magnetic contact inside a sealed glass tube and when a magnet comes near it the contacts get pulled closed
Because it's a sealed glass tube it eliminates the possibility of humid air getting at that actual contacts and causing oxidation, the only other thing I can think of to cause it to wear prematurely is arcing (electricity jumping across a gap, much like a spark plug).... which is a common problem with switches. Again, I'm guessing here (I don't know the Jacuzzi control circuitry), but when an inductive circuit (these thing usually control a relay which is inductive) is switched off, the electricity just can't stop instantaneously so a small and quick arc is produced across the gap, which in time slowly destroys the contacts. They can build up a resistance from the burning metal and in some cases even weld the contacts together.
To try and control and eliminate arcing a "snubber circuit" is usually built in to protect the reed switch. In the case of a DC current this is a simple diode across the relay and in the case of AC current (which I *think* is the jacuzzi case) there is a capacitor and a resistor placed across the reed switch.
Long story short... if you're shooting through a lot of flow switches then quite possibly your "snubber curcuit" on the board is fried.... OR... it's just all plain bad luck!
