i have a different theme now so it's not an hourglass but the same kind of sybmol comes up a lot now! when i'm sending messages, when i'm doing anything really, it just comes up and my phone chills for a few seconds, doing nothing.
my file free stays between 15-17.
my apps include:
twitterberry
google maps
flickr
soft reset
weatherbug
and viigo
if my file free gets below 15 i soft reset.
i don't leave applications running.
i only have one new theme downloaded besides the regular stock ones
i've even deleted some apps that came on the phone.
when i had an 8100 i had the same issue with it i don't know if it will help you or not but first of all make sure you are closing all applications correctly not just exiting them. and secondly i had my sms messages settings set up to show me 50 previous messages... when i lowered the number to the standard 7 it fixed it for me.