It's weird I had that problem too on my old system. I was using a 9800 GT and it was playing fine on the Ultra settings. There would be some lag sometimes but it wasn't my fault, it would be someone else in the game lagging. Then all of a sudden I was getting terrible FPS lag and turned down to high. Sometimes I think it might be small hotfixes or patches that fix a minor problem in the newer cards but aren't always tested fully with all the older cards. Was there a SC2 update recently? I'd recommend updating your drivers as it fixed my problem. Also I don't know if you let Windows auto update your graphics drivers but every time it has done it, it installs the wrong drivers or old ones still. I always go to Nvidia for graphics driver updates now.
when you say it made you unable to play certain maps, what exactly is making the game unplayable? You say you get 1 fps, yet next post you say you get 88 fps. So which is it? it's important to determine whether you are getting stutter (fps fluctuation) or latency lag (packets travel slow over IP).
also, which maps? are they particularly large? or is it random maps? custom maps?