[d8tabyte]

My advice? Check your logs, check which files are being accessed to identify the file being used for exploit, change all of your passwords and re-upload all core files. I would do a fresh upload - as in if you can remove ALL existing files from your host (or have your host wipe your account). You have a corrupt file somewhere that is being used as a backdoor IMO. It could be in an installed plug-in, an external program (i.e flashchat etc.) I would also be sure that you are the only one with admin access to your forums/ftp. Make sure all other accounts have limited access (if you have more than one FTP account) and any programs you are running beyond the forums - check for exploits on google. Ensure you are running the most up - to date software possible.

As for getting a new domain and a new host - unless it is a situation where the main host is compromised, that is an unnecessary step. You should work directly with your host - perhaps run a virus check on your account, and inform them that you are being compromised and they will do everything in their power to find the exploit. What host are you using?