Bit late to this (been busy all day), but glad my webpage helped
Yes, on occasion I've had T4 (AC128) go intermittent. Many would describe it as the driver, probably because many older text books did, but it's more accurate to call it the VAS (voltage amplifier stage). This circuit doesn't have drivers, as the VAS drives the output pair directly, but if there were additional transistors between those two points, they would be called "drivers". I'll get me coat!
In addition to the transistors and presets, some of the front end resistors can go noisy. I think I mention that in the article...
Sounds like several people have had a more productive day than me!