2007 pontiac G6 tranny slipping with rough idle

This problem just popped up on my
girlfriends car in the last week but it
got bad quick.  Started with the car
almost getting stuck in first and 2nd
gear.  It's an automatic transmission.  
First though was tranny fluid.  So I
planned on topping the fluids in the
next day or so.  We went out for
dinner about 45 minutes away and
the tranny started to slip much
worse.  Then check engine light
came on.  On the way to the store to
get tranny fluid I noticed the car
when in idle was idling very rough.  
Put half quart of tranny fluid in and it
seemed like it was improving but not
all the way better.  Then I put the
other quart in.

We were still an hour from home late
at night so not much I could do.  I
monitored it on the way home.  Car
was driving fine once it got into drive.  
But once I was getting down around
40 mph the car was running horrible
like it was struggling to down shift.  
Got home and the car in idle was still
very rough.

What's everyone think?  Need more
fluid?  Tranny filter?  Car was driving
great doing 60-75 mph on highway.  
Like I said problems were consisting
during idle and when the tranny was
down shifting out of drive.  It was
also slipping while accelerating from
zero.  Would the tranny cause a poor
idle?  If it's the fluid being too low
how many quarts are needed?  Like I
said I only put it one quart.