Starving Student pre/head phone amp channel fail when heated up

[detlefheck]

Hi community, I am a newcomer to building kits but have successfully put together two ACA amps and the Kork Nutube preamp. My latest project the SS HP amp and pre amp however has a problem. I use it as a pre amp. It sounds fantastic but when it heats up (about 15 min after it’s turned on) one channel, mostly the right, goes quiet. The headphone output still works though, and when I tested that I found out that inserting the headphone plug half way in fixes the pre amp channel issue. I have to leave the plug inserted 1/2 way in to continue listening on both channels. Is this a problem anybody has experienced? Is it possibly the headphone input that needs to be replaced? Any advice is greatly appreciated.