skipping in 99 tahoe

my 99 tahoe is skipping bad! the check engine light is flashing and it seems to have no power. it's going from 65mph to 45 to 60, and just feels like it's bogging down.
it's been at the dealership a week and they don't know why it's doing it.
can anyone give me any ideas on why it's happening?

Flashing engine light usually means an engine misfire.

thanks, i'm gonna ask about that.