My 97 850GLT (164K) began running roughly during a recent trip. Onset was intermittent at first--under acceleration loads only with check engine light illuminating and sometimes blinking. Soon it began to run poorly continuously. <p>Code reader shows that cyls 1-4 are misfiring. Removed cap and checked for cracks--found no visible discrepancies. Cap and wires are about 1 year old. Tried HEET in fuel with no improvement; replaced fuel filter with no improvement.<p>Can anyone suggest a logical diagnosis?<p>Thanks much.