1997 Chevy Monte Carlo black antifreeze

Went to get my radiator flushed and the mechanics said not to because my gaskets were blown going by my antifreeze being black. When I bought the car the head gaskets, water pump, plugs, and all upper gaskets had been replaced at 170,000 miles. The car has never over heated since I have been driving it. On the highway the car will sometimes hesitate while at a constant speed like the fuel pump is going out. This only started after having the air conditioner fixed a month ago, condenser, vacc, etc.
I'm not convinced a gasket is blown, any ideas? Been driving it for about 5 months and have had no other problems. Antifreeze has been black since I've had the car. No check engine light has ever come on.