OM Weather works fine in PR1.3 for me. It may be that you have somethings else going on. You may also want to check that you actually have PR1.3 installed (check the system->About N900 for the version). In some cases AppManager does the install, reboots, and you wind up in a mixed, partial install. (You can also run AppManager again and see if it still shows an upgrade available.)

The only thing I've found that legitimately won't work in PR1.3 is Xploder, from the OVI store. There are a few apps/libs that have issues, but not anything I use. (libcluter I hear has isues).

If you're going to re-flash, you may want to re-flash with 1.3 and update from there. Good odds you have some junk floating around from previous apps that's causing problems more than PR1.3 causing it.