power surge when under slight load and RPM's
 

99 f250 SD 7.3 International, 2WD. When Im driving under 40 mph and in 2/3 gear the engine surges almost like its going to stall, when I feed it more fuel it goes away, Started since I unplugged fuel heater, which was burnout fuses, Had a mech. tell me its the exhaust damper.
Did some searching and havent come across anything like that yet. runs fine at idle. and above 40 mph