Can you go over and look at it? Is the drop in RPM right away, or in 15 minutes, or just under load, when hot or cold, or what? If we had more info on when and how it is happening, it would be easier to come up with a possible problem.
If you can go look at it, I would let it run for 30 minutes or more at RPM, and not under any load. Look to see if it changes in RPM just free running.
If it runs fine, I would put it under a full load and time it to see how long before it starts to drop in RPM. If it happens right away, it is the engine, but if it take a while, I would think it is something simple to fix.
Try this. As soon as it does start to drop, quickly open the fuel cap. It may be nothing more than a plugged vented fuel cap.
If it is running fine under no load, and it runs fine for quite a few minutes under a full load before crapping out, I would have to think that the engine is okay, but I could not be 100% certain.
I am thinking that when it is running under NO LOAD, it uses little fuel, and the cap MAYBE venting just enough to keep it running at RPM, but when a full load is applied to it, its fuel intake increases faster than a half plugged vented cap can keep up with, and your RPM will fall.
This actually happened to me.