Sounds like you've lost a seal or similar on one of the rotors, probably the rear. Compression test it, but hold the button in while someone starts it, you should get 3 even bounces - i bet you don't.

unfortunately yes it sounds like rebuild time, but compression test it first - it could be a flooded rotor or fouled spark plug or something silly but youll be very lucky if it is!

compression test it/get it done!