1991 Honda Accord lacks power when warm

My 91 Accord automatic cranks cold with no problem, runs Great until the temp guage reaches normal operating temp, then loses some power. For instance, running 55 mph, tranny in D4 and has shifted to overdrive. As I start up a long hill. Start to lose power and find it necessary to downshift manually to make it up the hill. Also, car warm. Turn off. Run into store for a couple of minutes. Crank back up and go to pull out into traffic. And very sluggish and slow to get to say.0 mph to 40 mph.
Had several codes and replaced the following: oxygen sensor, speed sensor, fuel filter, pcv valve, has new EGR, distributor cap, rotor, plugs, air filter. Has new radiator, thermostat, radiator cap. I removed and cleaned throttle body, removed and cleaned idle control valve. Any help would be greatly appreciated. Thanks in advance.