http://forum.ppcgeeks.com/showthread.php?t=40609
Be aware, you must unlock your device before flashing any of the radios in that thread. Those are just for the radio upgrade itself, you are not flashing a new rom. I would personally suggest that you just try flashing a new ROM instead and see if it resolves your issue. Two names to consider, Juicy and MightyMike. Check on rom info in the HTC Diamond Upgrades section.