I just fought this on my 88. I replaced the thermostat, flushed the radiator and dug all the bugs out. I replaced the bottle and cap as it would boil over, none of this worked.
I finally went to Advance and bought a replacement radiator for 99 bucks. Problem solved, I guess after 240k miles I was due. Then that one started to leak but they had a lifetime warranty so I did it again no problems thus far. I wished I had done this earlier.