Husqvarna LGT 3654 with a K66 trans swapped into it 2 years ago. Kohler 7000 engine.
Ran fine last week. When going uphill yesterday with my mower deck engaged, or when mowing denser grass (6 inches), the engine RPMs would drop and it would almost stall out. If I switched off the deck then power would return so it's not the transmission.
Edited to add:
Also, there's a pretty sizeable loss in speed upon engaging the deck on a level surface, whereas it used to be fairly smooth about the transition.
I replaced the air filter and fuel filter, that didn't solve the problem. Spark plugs and oil/filter were done about a year ago, I'm gonna swap those anyway, but the oil color is fine and levels are ok. Mower belt is less than a year old and was not a cheap knockoff, it's the real deal. Battery less than a year old, maintained on a trickle charger. No issues starting it up except when I replace the fuel filter it takes a few cranks.
What should I look at next?