Issue with sylvania 6ca7

in my amp, with v11 VTA board running the SDS cap board, I have a 6ca7 - sylvania that biases up perfectly then after a minute runs away. It tests at 78 percent in my tube tester without issue. I've read a few sites that talked about these tubes being superior to anything new yet so many of them have this thermal runaway issue and it's more predominant in St-70's. Is there anything that can be done like changing out a grid resistor or is this tube simply destined for the trash? My jj's operate flawlessly.

Many thanks

Any socket it goes nuts, it came in a box of tubes I bought at a hamfest, probably why it was in the box.

try changing R29 - R32 to 150K rather than 220K or 270K as was original, that has helped me in the past with tubes with this type of problem

Without changing anything- I got another sylvania 6ca7 and put it in with the tube that was causing the issue. Last night I biased it up and it held stable for 15 minutes.

Strange.