I assume that you are checking the timing with a timing light and the mark on the vibration damper (front pulley)?? IF so, the most common cause of timing out of whack is a failed vibration damper. THis pulley has a center piece that bolts to the crankshaft and then the outer pulley sheave is isolated from the hub by a rubber ring that is molded in. When these get old and cracked, the outer sheave can slip on the inner hub, throwing the marks out of whack. You need to line up the engine on TDC and check the marks - or inspect the little rubber ring - if old and cracked, replace the vibration damper. THEN check the timing. IF still out of whack, time for ignition system diagnostics. THings that affect timing are engine temp (sensed on left side of head), knock (sensor on left side of block) and engine load/rpm. Knock (or something that the sensor thinks is knock) will retard the timing.