OK, time for an update. First the good news; the lens works perfectly now.
Now the embarrassing part; it always worked perfectly!
The problem? It disappeared when I removed the rear lens cap! The rear-most edge of the rear group was hitting the rear lens cap, preventing it from going to infinity.
But why? Well, just a minor detail... I was using an ETR rear lens cap. When I found the lens at the store in the junk section it had no caps. So I went to the crates full of caps and started to hunt. I couldn't find an SQ rear cap but found an ETR one and tried it out. And it fits just fine! I thought, lucky me! Only now have I discovered that the SQ cap is deeper.
And the most embarrassing part: I had the lens almost fully disassembled before discovering the source of my problem. I removed the front ring and front element (rear cap still in place) and when I checked the focus ring it still wouldn't go to infinity. Then I removed the rear cap, the rear element and the rear plate and discovered that the focus ring worked fine. So I figured something had been jammed. Reattached the rear plate, re-inserted the rear element and it still worked fine. Now before reinserting the front element I attached the rear cap, flipped it over and started to insert the front element. Tested the focus ring and "oh no! the problem is back!" "Must be related to the front element"...

Anyway this foolishness went on for some time until the light finally clicked on in this cloudy brain of mine! "I can see clearly now..."