I suspect the installation failure your experiencing, is related to the amount of available memory at the time of install (hence why a hard reset appears to 'magically' work). When you install ArcPad (or any other application) you need enough memory for: a) the installation cab to be copied to the device (temporarly), b) the contents for the cab to be 'unpacked' and then, c) the actual application files to be stored onto the device (then the temporary files removed). Any other applications and data on the device effectively crowds out this needed space.
During ArcPad 10 testing I installed and uninstalled arcpad on Trimble Geo's many many times successfully. The 'problem' hasnt been 'fixed', because there has not been a specific problem identified (yet!).
Regards, Marika