So to clarify Peter, its not a chug or the mower missing a beat, the revs simply slow down and then pick up where they should be.....hmmmm.
Ok, i've thought about it and i suspect one of 2 things. A rapid decline in revs and then picking up again(known as "hunting") would be caused by a dodgy governor spring. They are super sensitive on these models because of the lack of weight of the small 16" bar blade. Tweaking the spring/throttle plate mechanism sometimes helps but when all else fails a new spring should do the trick, but make sure you cross reference the model and trim code on the briggs website and get the right part number for your engine because sometimes they're different.
If its not hunting but rather just dropping off a little and then picking up again gradually i would suggest there may be a fuel/carby problem. Possibly contaminated fuel or a blockage in the mesh in the pickup of the carb.
After re reading your post and seeing that it only does it under load, then i would suggest there is probably very little you can do to fix it if the above 2 processes dont help. This is mainly due to the light bar blade setup and hence why these machines are targeted for users with small lawn areas.
