put some grease (if you haven't already) on the axle so you can tighten the nuts more or maybe your chainring isn't centered on the crank and you're chain is too tight?

Thx for the suggestions. I forgot about centering the crank. I read about it when I did the original conversion, but forgot to do it. It makes sense that the chain tension might look OK where I happen to check it, but be too tight in another part of the rotation.

Didn't know about the grease. I'll try that also.