Do you have a "Trap" like in a sink line?
I have a spray bar, and it has a "trap" right at the start of the bar. The same thing happend to me a few months ago...turned out that the PS was releasing small amounts of bubbles, and once there was enough which built up in the trap it purged itself.
do you have bubbles comming from anywhere else?
tighten up you lines with a big tie wrap, or a hose clamp, but be carefull not to over tighten.
if it's got threaded coupling, throw on some teflon.