Did you ever get a solution to this? We have the exact same symptoms. We've switched to the left side propane tank, but it was happening with the right side as well.
Furnace runs for a while (cycle on/off in normal fashion) and achieve thermostat temp. Works fine for hour or so, amount of time seems to vary. Eventually, with the blower running, it will start click-poof (ignitor then flame, repeat several times). During the click-poof it sounds like the flame is going, sometimes it may not be. Eventually it will stop trying to ignite, the blower will continue for a given period of time and then stops. The unit will not kick back on until the thermostat is set to "off". Turn it back on after 10 seconds and it will start back up and work fine.