My 2006 G6 had declining power as well as bursts of power NOW its top speed is 10 miles an hour. it has a bogged out sound and shifts gears all over the place. Also it seems to require coolant all the time with out any obvious leaks.
$hit, I hate to hear this, check your oil. PRAY to whom ever you hope can help that the oil is not white. If it is do not pass go, go directly to a shop. If the oil is ok check your exaust pipes while the car is running, see if an extra amount of water is blowing out. If so, do not pass go, drive directly to a shop.
It could be a blown head gasket. Very fixable, very dangerous too. I'm sure you know this I'm just clearing any doubt.