That is odd, without ubersampling I can easily keep 60fps at all times.
EDIT: Looking at graph the low FPS coincides with disk access?
Nah, I tested that too once before, most of the time TW2 occasionally uses like 5% disk, sometimes goes up to 30 but it's occasional spikes, SSD won't solve your FPS woes, it seems to be some other issue.
Just for the sake of it, I guess you should benchmark the HDD once just in case, read/write times should be around 100 or above(the latter mostly). But I highly doubt disk is the issue.