I'll take a guess and say it could be one or all of the following:
Carb out of adjustment-mixture
Carb out of adjustment-governor springs
Fuel pump (if equipped) going into retirement
Clogged passages in carb
Old fuel
Let us know specifically what mower you have, what brand and model engine is in it, etc. Some of these mowers have very specific cures.
#3
reddragon
sounds like carb.....maybe its time to go thru a complete tune up and carb rebuild