94 isuzu - stalling

My 2.3liter truck runs really rough and sometimes dies out at about 45 to 55 mph. I have replaced the fuel filter and changed the standard fuel pump to a electric pump and I am still having the problem. I changed the rotor and the distributor cap along with new plugs and still the same.
I did notice the plugs seemed to be melted on the tips.

what could it be???:banghead: