Well I hope you've enabled Clienthax's preloading models in which case you only need to convert them to .dat and don't risk the chance of screwing up the cache, if you use "add model ####" then I believe you need to conver it to a .dat and gzip it, you might not have to convert it back to a .dat but you must .gzip it.
Follow this thread by Galkon
http://www.rune-server.org/showthread.php?t=93086
It should be self explanatory...