A couple of furnace questions

[HighUintas]

I'm with Elknut on this one. Sounds like the flame sensor isn't providing the resistance needed to keep the valve open.
Only other issue is the "ignitor glows for a few seconds and then locks out". If you have an electrical problem has anyone checked grounds? Loose grounds cause electrical nightmares and these units are extremely finicky when dirty power is involved.

I'll order another flame sensor just in case that's it, but it never actually lights the gas when it has this problem. It doesn't get to the required 17s of igniter warm up for the gas to open. So I don't think it's the sensor.

I'll go through all the grounds on it today, but I'm pretty sure they're all good

 

[DanMan]

Doesn't sound like a flame sensor problem. Flame sensor only shows problem when a flame is present.
Really sounds like weak HSI. After 15 years it has to be weak.