The voltages will change when the pop/dropout happens, and letting us know which voltages moved around will help dramatically in determining what's going on.
As always, swap the tubes from the left side to the right side to be certain that isn't the issue. I would just swap them both at once.
If the pop stays in the left channel, use a wooden chopstick to poke around every connection in the amp while it's running and hooked up to an inexpensive speaker, and I bet you'll find a spot where you can poke the amp and make the speaker pop, and this will focus your resoldering efforts.
