What is the scenario that causes this?  I have seen it twice. One Sunn Concert Bass that did 112W into 8 ohms and only about 84 into 4 ohms (at clipping) and now I have a Sunn Concert Lead that will do again 30V into 8 ohms for 112W but only 16V into 4 ohms for a pathetic 64W at clipping.

Is this a scenario of driver transistors not being able to supply enough current to output transistors? I really don't get it unfortunately 

Sunn Concert Lead schematic is attached

Ok I have hit the wall and need assistance from the real pros   :dbtu:

This Acoustic 230 has weaker and weaker output over time.

When you turn the amp on output looks roughly like this


After about 10 minutes it will be something like this


It seems to be some sort of thermal drift problem because when the output is weak I can literally blow it back up by blowing on the PCB with output section on it

Q8 and Q10 have been replaced because I thought they were not able to drive the output transistors. Still the same. With new output transistors the result is the same too.  I have clipped caps in parallel over every cap in the output section and the result does not vary.

Here's a video of the output. It bounces all over and is not stable, let alone losing power over time. See how when I turn it up it has some sort of settling effect as it deals with trying to put out a big signal?
https://www.youtube.com/watch?v=Jq_UwCRaG6w&feature=youtu.be