OK, I'm a noob to this but I had a similar problem in my quest to tweak some things so maybe this is your problem.
Maybe your cache partition is too small. This is where apps and other ota things go before they are stored in data (ie, your application memory). That would certainly explain why you can't download anything, but you're showing enough memory.
It'd be worth a shot.
Credit goes to Firerat and Calkulin at Xda.
Install all of these files to your sd card:
[ MOD ] Custom MTD Partitions by Firerat (resize Cache, Data & System) - xda-developers
(in my files, I adjusted mtdpartmap.txt to work for a gingerbread stock rom (300 125) . You can probably get away with his mtdpartmap.txt just fine)
The instructions below work for Cwm Recovery v. 4.1. Calkulin's_FORMAT_ALL.zip didn't work for me so I did it by hand.
1) reboot to recovery.
2) backup (almost pointless, as your apps are gone. If you have an old backup, skip this step)
3) format cache, data partitions
4) install FR_Recovery_CustomMTD_v. 1.5.7b_for_EVO.zip (this will adjust your partitions in recovery)
5) reboot back to recovery
6) restore your backup or a new rom, but don't reboot yet
7) INSTALL FR_Boot_CustomMTD_v 1.5.7b_for_EVO.zip (this will adjust your partitions on your rom)
8) reboot your phone
9)...
10) profit
Let us know if this fixed the problem.
Sent from my HTC EVO 4G using Tapatalk