I used this guide and worked perfectly for me, try giving it a try. I never flashed before this was my first and it was a good guide.
http://forum.ppcgeeks.com/showthread.php?t=17667
looking at the guide I downloaded the sprint UNofficial release and then changed the .ex to .exe and it worked and installed great and then I ran the unlocker next and then download the No2Chem's Rom and opened the RUU and then placed the single file into the folder and opened the RUU and then flashed it from there and didn't have a problem.
What rom are you trying to flash it to?