I've got a buddy with a '99 K1500 RCSB 4.3 V6, it's stone stock and has about 150,000 on it. It is misfiring under load, especially climbing hills. It flashes the SES light and throws a code when it does this stating "Random / Multiple Misfires". We replaced the cap and rotor today and it didn't help. He put new e3 plugs in it about six months ago trying to fix the problem. I'm out of ideas any help would be appreciated.

in advance.