From my observations on the flight deck operating fanjet airplanes EPR falls during the takeoff as you accelerate.
Also to back this up I quote from the operating manual of a Boeing 747-200. In it there is a section that details why take off thrust must be set by 80 kts.
"Above 80 kts EPR will decrease due to the ram air effect. Do not compensate for this change."
As the aircraft accelerates the ram air effect increases the inlet air pressure and since EPR is exhaust pressure divided by inlet pressure the EPR reduces.