Caught him committing suicide a few times. He gets stuck on that walkway above the nuke, and hops off. If it doesn't kill him he hobbles straight back up and repeats. Possibly triggered from fast travelling from inside megaton. Planting medX and stims only delays the inevitable.
What freaked me out; I went to look for Walter (having seen him a few days earlier), and he wasn't there, so I walk out of the water treatment , and Paladin Hoss appears from there also with only 3 bars of health, WTH, did he kill Walter, so I go back in and look around, maybe there's a body....nothing. Weird.