I think you have two options:
1. Does your gmail and talk behave badly when you're off wifi and on 3G?
If no, then your problem is likely you've been connecting to ad-hoc wifi networks. The DLNA checkbox is acting as a workaround, as Android previously couldn't survive being on an ad-hoc network (your office IT can confirm this for you).
2. If yes, gmail and talk behave badly even on 3G, then it's time for more radical treatment, in my opinion.
Go into your privacy settings, ensure that Back up my settings and Restore is checked - then go to Accounts & sync and Sync all. Wait however long it takes for the sync to complete.
Then, go to SD & phone storage - do the Factory data reset.
When it comes up, it'll ask for your gmail account info. Skip any Bluetooth xfer, answer the gmail and other few questions. Your previous configs and apps will be restored at the rate of your data connection.
Ensure you go into System updates and update PRL, Profile, then HTC software update - and when it completes, update PRL and Profile again - and wait for your apps to re-sync themselves.
Google Talk - like Gmail or Market - are foundation level apps for Android, and no phone should be trusted without them working. Something is seriously goofed up on your phone (and it could just be a bit here or there locked as its opposite of a 1 or 0).
The factory reset ought clear things out and give you a clean slate and with the new backup/restore on google, it's actually pretty painless overall.
Don't bother to erase the SD card when asked.
If the problems persists after that, I'd take it in to Sprint.
Hope this helps!