well it could be alot of things .... Try these.
Reinstalling video drivers.
Reinstalling DX8.1 or 9.0.
Reinstalling EverQuest and make sure none of the files are bad.
Run a scan disk.
Do a virus check.
Reload windows.
*NOTE* try these things at your own risk!
I am not sure if it will fix your problem but try it and see what happends.
Now if your playing on 4.4 servers *DONT* reinstall EverQuest.
You might hear this a-lot but when you able to play again, make a back up of your EQ 5.* or 4.4 folder.
Anyway I hope this helps out. 