EPIRB :
Is it an M500 or M515 ? If it's a 515 I'll swap my Vx which Hotsyncs a treat
Seriously, try to track the source of the problem. You say you've tried a new cradle. Try these steps / answering these questions :
1. Did HotSync EVER work reliably ? If so when did it stop ?
2. Have you installed the Palm-supplied drivers for the cradle ?
3. Is the PSU for the cradle working ? I.E. does the Palm charge up when it's sat in the cradle ?
4. Have you tried HotSync via IR ? If so, did it work properly ? If not, what happened ? ( Instructions to set this up are somewhere in the Palm documentation ).
5. Is there a USB to serial adapter supplied ? Can you see if it works via the serial port ?
Give those a try then see what happens.