Thanks for the replies.
To elaborate a little further, it is happening with all the outlets in the shop, which are on 6 different 20 amp circuits. I agree it does sound like a bad connection, or execessive resistance somewhere, but whatever it is is affecting everything.
The builder came by today (nice of him to do that on a Sunday) and couldn't figure it out either. Took the fuse panel cover off and all the connections seem to be tight. We fired up the furnace and the 220V is working OK. So tomorrow he'll be contacting the electrician that wired the shop.