Miss fire when climing hill

I have a 2002 4.3l 4 dr S-10 with automatic trans.
Every time i go up a steep hill my service engine light flashes, according to the manual that means there is a miss fire.
It only happends when going up a hill for a while then stops and comes back on again.
What would make this happen does anyone have any idea..
I have replaced plugs, wires also put new fuel pump, and filter..
any suggestions would be helpfull.

Does the tranny have overdrive? If you downshift, does the problem go away? Any applicable trouble codes?

yes the tranny does have over drive, even when it is taken out of od it still does it.
it comes up PO300 cylinder misfire comes off when i stop but as soon as i start up the hill again it comes back on

P0300 DTC along with a flashing SES light is almost always caused by secondary ignition failures: Spark plugs, spark plug wires, distributor cap and rotor, ignition coil. Always use AC Delco plugs, cap and rotor in the 4.3L, they are very fussy. Put a dab of silicone dielectric grease on each terminal, inside and out, of the distributor cap. Post your results.

could be the knock sensor