Hi all, last year while mowing for 15 minutes to a half hour, the engine would just start bogging about every 5 or 10 minutes (not completely shutting down). Most times it would come back to full power in about 20-30 seconds. I ended up leaving the air cleaner off to see if I could catch anything visually while I was using the mower. By chance I covered the throat of the carb with my fingers and found that this would bring the RPM's back almost instantly and give me full power again. Seems that this happens more often under hotter weather conditions.

Check your fuel cap. Sometimes when they go bad it creates a vacum in the fuel tank and wont allow the carb to get anymore fuel. When you choked the engine it pulled gas harder.

Non-venting gas caps are FIRST suspicion for the symptom as Phillip says. BUT, if it's a 21" mower, can't say I've ever seen one not vent. You'd have to tell us more...engine numbers or application, and age. Older B&S with poorly seating intake valves would produce the same symptom.
Paul