Things to try:
Trash the following...
HD > Users > Your Account > Library > Caches > Safari.
~/Library/Preferences/com.apple.Safari.plist
If those don't do the trick then dump the whole ~/Library/Safari folder (but take out the bookmarks.plist and save it somewhere first)
Check the Safari.crash.log to see what is really happening.
Sometimes fonts can be the reason. Open Font Book and check for any duplicates or possibly any new fonts that may have been added by the change to the new ISP. You can also validate fonts within the app.
Download and resinstall Safari again.
Here is some advice from Apple:
QUOTE
If possible, try connecting your Mac to the Internet via a different ISP. If the issue occurs only when you are connected to a specific ISP, contact that ISP for further assistance.
For various reasons, your ISP may cache Internet connections to minimize traffic outside of their own network. This cache can be compressed then decompressed with some alterations when the packets are delivered to your Mac. Tolerance of this may vary by web browser--third-party web browsers may respond differently than Safari.
Good luck with that.
Create new user and see if it still crashes.
If any of these do work, once Safari is open, go to Safari menu and select Reset Safari to help clear things up.