2001 Dodge Caravan 3.3L Ignition timing issue

Symptoms: occasional failure to start, random rough running, random stalls, bucking at highway speed, occasionally stalls/runs rough in reverse, OBD monitor shows ignition timing LATE whenever symptoms are present. (Normal idle about 18 deg BTDC, in motion 35-45 BTDC, when symptoms present timing runs from 6 BTDC to 2 AFTER tdc) Sometimes goes a week no trouble, then may be a real problem for a few days, then fine again. Doing it more than a year now, does not seem to be getting any worse (or any better!). Fuel press 58psi at rail