code 0300 flashes under load

2001 c1500 4.3l. running at idle is fine. only when driven or put under load...misfires occur in all except cylinder 5 according to the scan tool. i have replaced cap, rotor, and plugs (all ac delco). still same problem. check wires w/ spray bottle and did not see any arcs or notice a difference in idle. funny thing is...that while in park...when i give it gas up to 3.5k rpm, the speedometer jumps up to about 3-6mph. i don't know if its a faulty pick up coil. when driven up 55mph...the ses light starts to flash. when driving below 55mph...ses is off. anyone run into this kinda problem before?

The ses flashes usually when there is a misfire present. If you read the manual about the light flashing it will tell you to drive it as little as possible and get it serviced as soon as you can. They say that because there is a greater possibility of damaging your Cats if you keep driving it with a misfire condition. Your misfire condition could also be being caused by dirty injectors or fuel starvation possibly by a clogged fuel filter. Just a thought.

well...if the system is running lean...that shouldn't cause to much damage to the cat right? my understanding is that if the engine runs rich...then i should be concerned w/ the at. ill look into changing the fuel filter and see if that helps. thanks for your input.

You say all cylinders except #5? So you are getting the following codes: P0301,P0302,P0303,P0304,P0306,P0307,P0308? If not then please explain how you scanned, what scanner you used and if you did a drive cycle with tech scanner recording while you drove.

a buddy of mine has a otc genisys scan tool. he was watching the 'missfire data' and saw all except cylinder 5 miss while i was driving. the only code is 0300...only when driving highway speeds (ses will flash). once car gets below 50-55...the ses stops flashing...but there's still missfire.

Running lean will also burn up your cats due to excessive heat.