help

I have a 97 GMC vortec V6, it has 222,000 miles. I have been having problems with it for the last three years. The service engine light has been on for a while and giving me fits. The truck will missfire quite often. It gives a P0300, 0339, 0340, 0420. I have been watching and working to get this under control, but no luck. When I drive down the road I have noticed that as I go down hills the truck will shut down ( ex: the rmps will read 2000 in 5th gear then drop to zero for about a second or two then restart), usually back firing through the exhaust or causing the truck to jerk bad. I have changed Crankshaft sensors, plugs, wires, rotor, distributor cap, then complete distributor, fuel pump, all filters, oil changes regularly. The truck is still in great condition, however, the electrical problems are a pain. Thanks

Welcome to AF. First thing you need to to set is set the cam synch signal using a scanner capable of reading this, try and get it at 0 by moving the dist adv or retard as required, then run the engine at 1k or more to set the signal. Confirm on the scanner you are recieving the cam sensor signal of course.