same problem
i have exactly the same problem but i have a HTC Desire-S. It times out every 5 minuites which is annoying when your buying or downloading something.
when it does it you have to refresh it by pressing the button on the top of the phone to get it off standby and then unlock the screen and it comes back on straight away. (and it lasts another 5 minuites) you can see the little 3G icon at the top flashing as it searches again. also, i THINK, when you tether it (by connecting it to pc with the usb cable) i don't think it times out and speed is stronger. Also when on the portable wifi hotspot page on the phone, click on menu then click on advanced, click power mode and click "always on" try turning phone on and off incase it's that.
theres one thing i still need to try that might be the problem- a setting in the sleep mode of the phone, like one of those "after 10 minuites of inactivity" things that happens after sleep mode where it goes into power saving, it might be programmed to turn it off to save power, please post it if you find the answer though. i also havn't tried if it's the "always on mobile data" in settings
theres a lot of posts about this problem with HTC, people need to report it to them.