1993 5.0 ho mustang Hesitates under load only when hot?

My 93 does the same. Once it is hot it will get up to 3k and bog down. If I don't let up on the accelerator it will backfire and take off. The low coolant light came on briefly, but the radiator is full. The temp reads really low. Thought it might be an open thermostat, but I have good heat.