problem with Ultralife CO2

I've been having a weird problem with an Ultralife CO2 kit.  Any insight
would be appreciated.

The unit is set up as directed with a pH controller attached.  It appears to
work just fine most of the time, but then several times a day it *blasts*
CO2 through the system for several minutes.  It appears to then slow to
normal again.  I recently added a Dupla bubble counter to the system and
re-set both the large control valve and the little "fine tuning" valve so
the bubbles would come out at about 10/second.  Worked fine for two days,
but today I found it blasting again.

Asssuming no one else id fiddling with the settings (which isn't a sure
thing), could I have a defective regulator or something?