I have a 4.5 hp Briggs walkbehind that is 3 yrs old. Never had a problem with it till I started it recently. When mowing, it acts like it is going to die, like it is gasping for air or something. I emptied out the gas tank and changed the oil but still the same thing happens. The other day I mowed till it ran out of gas, about 5 minutes before it ran out, it acted fine, full throtle. The kind of mower I have is a side discharge with a mulching option. What could be the problem ?