I have a 93 civic dx hatchback 1.5 with a manual trans. It runs excellent when it is cold and even when it is warm it runs good. It seems like when the thermostat opens (normal operating temp) it starts to hesitate but only when I take off or really get into it. It idles fine but it will hesitate even with it parked and me manually moving the accelerator. BUT ONLY WHEN IT IS AT RUNNING TEMP. I don't get it can someone please help.

cold = good running
warming up = good running
normal temp. = bad hesitation