I had the same phone and the exact same issue, now i have the sensation xl, its all to do with
the lack of internal memory. A lot of the bigger apps cannot be saved on the SD card , I.e.
adobe reader, facebook, flash player, YouTube etc. So I found myself deleting the facebook
app as It was the biggest waste of memory and constantly having to clear the data on each
app. If you go to settings > applications then click on the biggest data using apps, then 'clear
data', this usually frees up some memory.
Also have you done the android 2.3 update for your desire? It's a manual update that
effectively debrands the phone and clears a lot of the shit off it, only problem is it deletes all
your apps and contacts and stuff, so you'd either need to back them up beforehand or just
install them all again.
Here's a link with a guide on how to do it:
http://m.techradar.com/news/phone-an...oid-2-3-986502
I did this when I had my desire and i found it helped a lot