My 2005 CTS-V

[TheBlizzard]

Yeah I was just going to say check the voltages under load, you could have a loose connection or a failing boost a pump like you mentioned. Weird issue though.

[C3L1CA]

Do you still have it hooked up to the little dial switch?

Mine would go lean under boost and thought it was my maf at some point, turns out those dials go bad.

Try cutting the wires that go to the dial for lean/rich and solder them together. This will cause the boost a pump to run at full voltage all the time and fixes the dial switch problem.