If the lens barely works, then it's just not a good lens. Period. How do you take something that works half of the time to a paid gig- heck, or any gig.
What's more sad is that it's a 20yr old piece of shiz design. Too many canon apologists over the years is probably the reason they have yet to produce a new version. I actually tried out my nifty fifty a few days ago, and no focus issues (never did have any).
But seriously...Twenty years old and they haven't even upgrade the build materials. That is beyond pathetic IMHO
At least Canon didn't degrade it as opposed to the 50mm F1.8. The mark I version is so much better.