Missing and light dimming

[Mathews825]

I have a 1999 sierra with a 6.0. It has a really bad miss and the lights dim and the volt gauge drops from 14.1 down to about 9 when the rpm's get over 2500. but if i turn the tow haul on it will go up to 4000 rpm's before it does it. i have changed the battery, alternator, plugs and wires. I still getting upset and i just want it fixxed. Any ideas?