P0300 Code '96 Sierra 5.7L

I have a 96 GMC Sierra Ext cab Z71 with a 5.7L v8, 109000 miles. The engine seems to misfire only when I am traveling fast on the hiway, esp. up and down hills. The SES light will come on and give me a P0300 code, (mult. misfires on random cylinders). I have changed the air and fuel filter and ran some fuel injector cleaner through the tank. I changed the plugs, wires, rotor and cap at 88000 miles. Any help would be appreciated.

Check distribuator base where cap screws to. Had one broke on me and it caused a miss fire and bog out.

I have a 96 GMC Sierra with the 4.3L, That is now doing the same exact same thing and the same time as jaksdad66. I looked over the distributor and everything looks fine? any other suggestions/answers would be greatly appreciated.

I have a 96 GMC Sierra with the 4.3L, That is now doing the same exact same thing and the same time as jaksdad66. I looked over the distributor and everything looks fine? any other suggestions/answers would be greatly appreciated.
I replaced the distributor rotor and also found the wire between the coil and distributor cap had a loose connector. Hope this helps.