1992 GMC Truck

Engine Performance problem
1992 GMC Truck 6 cyl Two Wheel Drive Automatic 120000 miles

one time engine will start and run fine, next time engine may not start and next time engine may start but sounds like timing has jumped and engine knocks loud and runs rough. Changed plugs-cap-rotor-coil-fuel filter-pcv valve and it ran good for a couple of months but problem has started again. Any ideas?
92 work truck
February 27, 2010.

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