"Our manuals are cr@p!!!"
I know the feeling!
According to my 747-400 notes, when the aircaft is operating in VNAV Speed mode, the A/T computer (which is contained within the Flight Management Computer on a 747-400), calculates a desired TAS value and compares this with the current aircraft "airspeed" to generate a difference value. This difference value drives the throttle levers. I know TAS is fed to the FMC from the DADC's (probably along with lots of other things), but I can't state unequivically that the "current aircraft airspeed" in my text is _TAS_, but this would make sense. So I can see how TAS might affect A/T engagement in at least one A/T mode.
Does your 757 have digital engine control (FADEC)? If the EEC's are using TAT data provided by the TAT probe(s) mounted on the airplane nose, this may also be a factor in A/T disengagement.
Anyway....Hope this helps.
Cheers.
Q.