Have you run the car in park with the radiator cap off? You said the first time it overheated the coolant was brown-like chocolate milk. Sounds like radiator is rusting.
Take off cap and let run. Looking towards the firewall of the car, you should observe good flow from left of radiator to right, if not, could be clogged, also, large amts of rust can be indications of clogged radiator.
Has the coolant got brown since last flushing? Also, the second time it overheated, did temp gauge still read normal?? If so, you will need to change either or both sending unit and/or temp guage.
Doesn't sound like radiator is cooling coolant efficiently.
Previous owner of car doesn't seem like he cared to much for her either.
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'79 Mustang Coup w/ 4" Cowl
Stock 5.8L, C4 w/ shift kit
Holley 750 CFM, Edelbrock Performer RPM intake
1 5/8" MAC shorty headers, Al driveshaft
2.5" Off road H-pipe, 2-chamber Flowmasters
Front: 225/60/15, Rear: 255/60/15 Eagle GT II
Weld wheels (15x6;15x8), 8.8" Rear end w/ 3.55 gears
14 x 4" K&N air filter (getting the Xtreme setup soon)
"Red, thou art my companion. Hasten now your quickened metamorphosis to Green that I may conquer all who dare abide there beside me. May they be left thither behind burnt black." ---Fox Body