Air filter good?

Air filter is good, spark plugs are good. It just seems to slow down and will sort of pulse but when the blades are disengaged it runs fine with no missing.

Check the vent in the gas cap and make sure the fuel filter is flowing OK.

Have you made any changes or tuned up the engine lately?

I'd look toward fuel filter being "almost" clogged. Under load, the engine requires more fuel than can pass through the filter. Carb bowl starts to run out and engine slows. Bowl is replenished a bit and engine starts to speed up, repeat.

Now the 30 minutes part points to a clogged gas tank vent.
Try loosening the cap momentarily after about 20 minutes. If it only goes about 10 more minutes before it bogs, it's probably not the vent.

You may be having a combination of both contributing to the problem....