It's possibly starving for fuel. If it has an O.E.M. inline fuel filter, you might try replacing it with an automotive filter so you get greater capacity for storing the stuff that a fuel filter catches before becoming "stopped up". I like the clear plastic kind so you can see if it has trapped water. I think you probably have 1/4" fuel hose so getting a nice, robust filter at a local parts house or even Wal-Mart is no trouble. Pay attention to the directional arrow pointing the right way to install it. Don't worry if you see a pocket of air in the top of the filter that won't go away, that's normal and does not mean your are sucking air or anything. The reason I suggest it may not be getting enough fuel is that you say it starts well and runs well until you try to cut grass. If it isn't acting strangely otherwise, it might not be getting sufficient fuel to meet the engine's needs when the real load is handed to it. Go out there and try to mow some grass the way you did before. Do this for about 15 minutes and then cut off the engine right there (so you can read the evidence on the spark plug tips without running the engine without being loaded) Let it cool down a bit and remove the spark plugs to inspect the tips. If the tips are dark black and sooty looking, you are getting too much fuel and/or not enough air. In that case start looking for a restriction in the air induction system (air filter and check any ducting leading to the air cleaner for foreign material, like a mouse nest). If the tips of plugs are white in appearance, you are not getting enough fuel and the engine is running too lean.

You are correct, "generic" certification is often meaningless. ISA (International Society of Arboriculture) certification is the gold standard. Certainly not all ISA certified arborists are equally qualified, but at least they tend to be consistently better than the guys that think they are qualified because they know how to get their Homelite saw started. Whether you need an ISA certified arborist to take down a tree this size depends. If the tree could fall and damage something, it's a good idea to get someone you can be confident in. If, however, the tree is just out in the middle of your yard, it doesn't look like much of a challenge and probably the guy that can get his saw started could do the job perfectly fine.

Sorry, the picture is on my other post. Sorry for the screw-up.

I guess we all have a problem then. Welcome aboard. Dahlia are not that difficult at all to grow. You can expand your collection as your comfort level rises.