Thanks for the help Gustavo. Unfortunately I don't know anyone nearby with a 990. When I do the test I do hear something - sounds like a solenoid or something. Is this what the cycling sounds like? Also, how do I distinguish a problem with the pump itself from a problem with clogged filters? I assume that clogged filters result in a gradual loss of pressure and hence a gradual loss of performance. What happens when the pump itself dies? Does it die all at once, or does it gradually lose pressure? I'd rather not replace the pump if it isn't necessary.