Small Engines (Lawn Mowers, etc.): Chugging problem, briggs straton, fuel tank cap

Question
I have a 12.5 Briggs & Straton 12.5hp lawn mower (make is a Lawn Flite. The engine works fine by itself. When i engage the blades it is fine in 1st and 2nd gears and also in 5th and sixth gear, but it chuggs like crazy in 3rd and 4th gear. It has to be something easy to fix? Can you point me in the right direction. Thank-you in advance!

Answer
Brad
Here is some trouble shooting tips

Problem: Engine operates erratically, surges, and runs unevenly.
Possible causes:

Fuel line or fuel tank cap vent is clogged.
There is water in the fuel.
Fuel pump is defective.
Governor is not set properly, sticking, or binding.
Carburetor needs adjustment or service.
Loose carburetor/intake pipe resulting in vacuum leak