Post-Speedball install: pop and then no sound in L channel

[denti alligator]

I haven't had any troubles with the Speedball, but this week about 4 minutes after turning it on I get a loud pop in the left channel and then no sound. LEDs are lit. Yesterday I turned off the amp, took out the tubes, put them back in again, and repeated this step about 3 times before the amp was back to normal and I got one hour+ out of it before going to bed.

Tonight I'm not having as much luck. Yet. Any ideas what might be causing this?

EDIT: got it now, after another loud pop the sound came back! We'll see how long it lasts.

 

[Doc B]

Bad solder joint

 

[denti alligator]

OK, will re-flow them all.
Strange thing is that this time, after about 15 minutes, the R channel slowly faded out to silence.