Old DR AT1 walk behind mower - problem with blade clutch

[baskervi]

Last year, I ran into a problem with the blade clutch - when I'd attempt to engage it, sometimes it would bog down the engine and not spin the blade. I'd try it a few times, and eventually it'd go. I've disassembled everything, down to the three ball bearings in the clutch. Everything seems reasonably clean, the bearings seem to spin OK, but I'm still facing the same problem. Has anyone had a similar problem and figured out what was going on? Thanks