First thing I'd try is a regular CAT 5 Ethernet cable to rule out a problem with your VDI cable. Just be careful plugging and unplugging the ends as it won't have the protective sleeve.
I also use JTV69 without battery, but I haven't crossed this problem, yet. Did this problem started after you did some firmware update (JTV or HD500)? If so, I would try to reinstall latest firmwares for both units.