What would cause a bad mis that wont let me go over 30mph

After pulling a trailer with a ground issue my 5.9magnum started acting up
when trying to go over 40 mph, it now won't go over 30.  I've replace plugs,
wires, coil, tps, computer relay fuse, checked fuel pressure (50psi) and
injectors.  Runs smooth at idle until you touch gas and starts to cut out and
die if you give it to much.  Cat good and nothing in oil, water, or coming out
exhaust.  Any suggestions???  Thanks.