Phone memory full and it shouldn't be

I have a weird problem with my android phone: for some reason, I'm getting a 'Low phone memory' error message, and before getting the message the phone was slow and unresponsive. I have 2 GB internal storage; in the app manager, it says 1.2Gb used, 27Mb free. Where did the rest of the space go? I noticed this slowdown after installing an app (QR droid) from that Play Market (or whatever name it has now). When I first saw the error message, I had like 100 mb free in the app manager, and each time I reopened it the space was decreasing. The phone is a Motorola Defy, rooted and using Miui, android version is Gingerbread. So how can I fix this issue?