Howard 350 Problem

Hi All!

I've just purchased a very sorry Howard 350 from the company I work for. I've used this machine countless times before and It has now developed a fault, which I'm hoping someone on here maybe able to relate too!

She drives fine, all 3 gears work and the engine is performing well when she is in free motion. The problem lies in that when the blades are engaged. They turn under power but as soon as they meet the ground they disengage and there is a grinding noise from within the gearbox assembly.
After downloading the manual (thank you forum for these resources!) I'm pretty sure it's going to be linked to the rotor selector gear. The lever itself stays in position so I imagine something on the crankshaft is slipping?

Has anyone come across this fault before and can offer some wise words for repairing?

Many thanks to everyone,

Jeff