For those who did not wait for Sprint to release the official Android 2.2 Froyo build and had to download the unofficial, non-final build of the Flash-supporting ROM from HTC’s servers, there is a fix in tow for your HTC EVO 4G. However, the fix does require a full wipe of your device, but it should rectify any instabilities you have by installing the official version. This will require you to be tethered to a PC to install.
The fix is available over at XDA-Developers and you can download the .EXE file from the links there. Directions are also given over at XDA.