Civic Oil Pressure Light

During colder weather the oil pressure light on my '90 Civic LX will flash on and off or stay on for periods of time seemingly unrelated to how warm the engine is. It doesn't happen during warmer weather. I have changed the oil, oil filter, and oil pressure sender with no obvious impact. The oil level is full or near full when this happens and there is no overheating of the engine that I can detect as well as no exhaust smoke or other indications that something is not right. Any ideas what is happening and what can be done to fix it? Thanks....

Could be a bad ground if the level is full
http://www.binghamtonlife.com/manual/2gsm/images/91-23-114.jpg
http://www.binghamtonlife.com/manual/2gsm/images/91-23-88.jpg
Welcome to AF!