Jon,
Adjust the cold weather switch, as suggested. (this is in the Pat-Trap manual which can be viewed online, http://www.pattrap.com/manual.html ). If the problem persists then you will probably need to replace the throw arm valve, not a difficult task. You can watch the operation of the throw arm and where it stops, if the stopping position is varying and sometimes firing thru, then its the throw arm valve. Also, always feel free to call Pat-Trap, their customer service is excellent and any of the people answering the phone builds and works on the machines on a daily basis. From the front of your machine look on the right edge of the top plate for the serial number. If it starts with SW that's your model (series), else it starts with G and you have a G series.
Also, P-Traps will run on many different lubes, the very first machines were shipped with a 50-50 mix of kerosene and ATF. Presently Mobil 1 5W-20 is recommended.
Jim