Im also suffering from this problem, the usual response is battery degredation - your battery must be old.
As im sometimes away from power for a while i have three batterys, battery 1 came with the phone, and the other two are less than 12 months old, all of them are official motorola batterys, and all of them exhibit the 30 to 19% battery drop with CM7, this problem presetns itself even with wiping the battery stats, both in recovery, and using the wipe battery stats app.
Ive also been trying CM10 for the last few days, and that does not have the problem, the battery level successfuly goes down from 30% 29%,28% and so on, on a nice level slope in the battery usuage menu, with no sudden drops
, however cm10 does have the problem of increasing lag as time goes on.
Im just about to go back to CM7, however as a test i will try a clean re-install, wipe /factory reset from recovery, then titanium to get the apps back, as opposed to the restore option from recovery.