Chances are, your clutch may be slipping. This is a problem that should be repaired by a professional. You can also check out if the brake is engaged while cutting. If it is, it may simply require to be released. You should also examine the fuel and your spark plug.
Wow at the other answers. Lol. When it loses power does it run better if you put the choke on? If so it's probably a carb issue. If the choke doesn't help it's possibly a scored cylinder. When it warms up the parts expand it gets tight and will appear to lose power