  We report the discovery of two long-term intermittent radio pulsars in the
ongoing Arecibo PALFA pulsar survey. Following discovery with the Arecibo
Telescope, extended observations of these pulsars over several years at Jodrell
Bank Observatory have revealed the details of their rotation and radiation
properties. PSRs J1910+0517 and J1929+1357 show long-term extreme bi-modal
intermittency, switching between active (ON) and inactive (OFF) emission states
and indicating the presence of a large, hitherto unrecognised, underlying
population of such objects. For PSR J1929+1357, the initial duty cycle was
fON=0.008, but two years later this changed, quite abruptly, to fON=0.16. This
is the first time that a significant evolution in the activity of an
intermittent pulsar has been seen and we show that the spin-down rate of the
pulsar is proportional to the activity. The spin-down rate of PSR J1929+1357 is
increased by a factor of 1.8 when it is in active mode, similar to the increase
seen in the other three known long-term intermittent pulsars.
