1988Mustang

i have a 88 gt its all pretty stock just have cold air intake and flex fan... when its hot out my car decides to act up yesterday i drove bout 40 miles and i was fine but then i drove bout 20 more miles and it started acting up almost like it was running out of gas bogging out and all and had jus about no power then the car sat for a while and the motor was not hot any more so i drove about 15 miles or so and the car started acting up again i have no thermostat in it but the temp gauge was runnin jus barely above the first line after 130 mark but then when it started cooling down at night lil bit it rides fine got no problem any help in some sort of direction would help alot thank you