why does my car change its timing without provocation?

[E Dogg]

First, a lil backround on the the carit is a 91 240sx with a 98 ka24de block and head. All the manifolds sensors are the stock 91 ones. Hotshot header with test pipe and 2.5 "exhaust, CAI. I set my timing to the stock 20 deg below tdc and the car changes it anywhere between 30 below to 5 below at randomI did try a new knock senor but no avail- Ideas anyone?
Modified by E Dogg at 1:50 AM 10/18/2004