1966 wiper arm stripping problems

[Rust-O-Matic]

The original wiper arm/transmission linkage setup on my '66 was in pretty bad shape, so I recently replaced everything. New transmission arms and knurled shafts, as well as new wiper arms and blades from Classic Parts. Everything seemed to work great until I drove it in the rain today.

Once the windshield was wiped clean, the friction of the blades against the sorta-dry glass was enough to make the knurled transmission shafts start popping and rotating independently of the wiper arms, messing up the splines inside both new arms. I shut them off and popped them back down 3 or 4 clicks once I got where I was going. I used them very sparingly on the ride back home. So it looks like as long as the glass is wet, they work without popping/stripping. This isn't gonna cut it though.

I'm super bummed out now. I thought I finally had working wipers again. Has anybody else experienced this? Anybody know what I can do to fix it? The splines on the knurled transmission shafts appear to be ok. It looks like it is damaging the splines inside the wiper arms, like maybe they don't fit together real snug. I thought about using a little adhesive of some kind to hold them together better, but it would be a major pain to remove next time the cowl has to be removed.