This is totally what I am experiencing on my bonni.Exactly to the tee!
I have tried everything even having the tcc checked out but no one and I mean no one can find out why I am having this problem seems like a occuring problem with alot of bonni owners.
I have changed the spark plugs,wires,egr valve,coils,o2..both..,Had the fuel pressure checked,did the boost bypass to no avail and switched it back to normal,had the trans checked,had the pcm scaned,god I think I covered everything to check but still the chugging going up hills at a steady speed,I even changed the cat to no avail.
I have just learned to ignore the problem and figured it is normal operation just like gm said to me...I have been to 3 gm shops and they say nothing is wrong so I figure gm knows there cars so this must be normal....YA right!
I sure would like to know why this happens to our cars and how to fix it!