Surging

1 st timer here. I have a 1992 c1500 4.3 V6 TBI fuel injection. 3000.000+ miles on it. This thing is driving me nuts. For the last 18 months after I have been driving it and at 190 - 200 * temp. St times it will start surging or act like the ignition is shutting off for 1/4 to 1/2 second. As long as it is at a steady speed it will surge. Give it a tad of gas it`s will stop surging or and up grade in a road it`s fine try to drive at 55 mph surges like mad some times after 20 minutes some times after 1/2 hour. I replaced the fuel pump and gas filter no change put in a used EGR valve no change put in a new EGR valve selinoid no change. Put in a ner dist modgel (sp) no change stuck in a used dist. Same thing. New TBI base gasketthe large ring just under the rotor is not easy to turn like old dist. No idea what that part is called. New vacuum lines new PVC valve. I have sprayed around the intake manifold no change in rpm once it did by the EGR valve but not again. Every once in a while the service engine soon light will come on. Checked codes with jumper wire shows only code 13. The truck will restart after normal temps but like the ignition is advanced to much or a low battery no matter where the timing is it`s the same thing. I do know I need a new radiator for it it is pretty rusted up in side. Not losing any fluid in radiator. Don`t know what to do. Can not afford to buy another used truck. Any one have any ideas? This needs to last me a year or two longer