Curt, I don't know about projectors with Sony 09MI tubes, but my friend had also this problem previously with a red. He replaced it twice. It was clear: air entered from the front glass!
And when I saw a bare tube out of his housing I immediately understood the reasons for these failures! I never saw this construction on other tubes, the frontal glass with phosphores is glued on the other part of the tube. After so many years I think it's not anymore reliable! This is what I guess.
On this red we saw glycol on the phospores, but NOT with the broken glass as sometimes happens with fully sealed tubes (P19LCP for example).
With the last blue, for about half an hour, pj worked well, then we heard HV discharges into tube.
Note than before this pj worked for 3 years perfectly well. It's ok!
Unfortunately with last blue neck, board was damaged (I can see resistor R152 well burned), if I connect this to a burned tube I get full light! Have to turn off in 2 seconds!
So now we need a "Panasonic" neck board R762292, if I remember well, or to fix old amp and convert it to Panasonic. Experience with Sony tubes is ended and we're not anymore able to try with others. Panasonic is last change for this pj!