Some sensor that is an input into engine speed control (i.e. mass air flow) has an intermittent signal that their software is not flagging as faulty

 

OR the transmission is slipping.

 

Thanks and sorry for the series of posts.. would not work in one post for some reason.

 

Could it be that the tranny is coming out of lock-up and back in again? giving you the engine speed changes.

 

Shawn,

That's one of the areas I wish they'd test, but so far they have not! They've been focusing only on an ECM and engine speed signal.

If that is what it is doing, then why would it do that?

Thanks!

 

This does sound like a torque converter lock-up issue. On most GM vehicles, transmission goes out of lock-up thesecond the brake pedal is depressed, so a faulty brake-pedal switch (the part that senses when the brakes are applied, right above the pedal itself)could cause this. Did you also notice cruise control disabling itself when this happens?