Reduced Engine Power 2008 Grand Prix

I have a 2008 Grand Prix. for the last two
weeks or so the reduced engine light has
been signaling I can go as fast as I want
but it takes a lot longer to speed up the
RPM's don't go over 1500 and when I shut
the car off and turn it back on the whole car
will shake. I took it into the shop but they
said everything looked OK with the
computer. I don't understand why it
happens so randomly I can be going 60
miles an hour or 20 and it just happens
randomly any ideas or suggestions would
be greatly appreciated. I don't know what
else to do. This is the only vehicle I have
owned I have had it for 4 years now and
this just recently started happening.