briggs and stratton 375 classic?

i have a briigs and stratton classic 375 powering a rotary mower and it has done very little work, the mower started hunting so i replaced the govorner springs and that didnt cure the problem!!!

i then changed the carb gaskets between the carb and fuel tank this made the engine run fine after hunting for a couple of seconds when started after that the engine runs sweet until it is put under load and then it starts hunting again.

i was thinkin the springs might have got damaged when i changed them and it didnt cure the problem any body have any ideas?????