I had similar problems with my WAG54G. Had my Jetbuster connection running for a month the the problem started.
This is what I did:
Flash to firmware 1.02.7 just like you did, made router much more responsive and stable (it used to be extremely slow loading admin pages but no more). But the connection still keep cutting off.
Made hours of call (yeah, it took Slingshot 1 1/2hrs to pick up the damn phone) to Linksys and Slingshot, but of no help.
Finally figured it out, it was the dsl line voltage!. For some reason the voltage dropped, maybe more connections were added to our node. All you need to do is just add a line filter for the dsl connection to compensate on the voltage.
Let me know if this works
Cheers
Leon