I drive a 2005 silverado 4.3l v6 manual transmission 82,000 miles. a few weeks ago i was geting on the freeway in a bit of a hurry so i was shifting at about 3500- 4000 rpm when I switched into 4th gear at about 60 mph i noticed a significant loss of power and my truck struggled to reach 70 mph ( at half throttle) at a little under 3000 rpm and when i finally reached 70 my check engine light began flashing. i got scared so i let my foot off the gas and went back down to 60 the light flashed for about 10 seconds and turned off on its own i took my truck to a shop to find out why the check engine light came on. they ran a computer diagnostic and it showed "P0300-Random/Multiple Cylinder Misfire Detected" the only other information they could give me was that although i had about 2-4 misfires on cylinders 1,3,4,6 my primary problem was cylinder 5 where i had 676 misfires . i was told to check my spark plugs so last night i changed all the spark plugs,wires, and distributor cap. after i did this i took the truck out for a test run to see if i had fixed the problem so i repeated the scenario that made my check engine light flash the first time and the light began flashing again. Can anyone please help me figure out whats wrong or what else i could check/replace before I take it to a shop and spend tons of dollars in repairs, i hope i gave you guys enough information please let me know anything you think might fix the problem