Half dead with codes P0300, P0304, P0305, P0306

Hi all,
 
My ML has no power, had to have it towed home today.  It has been giving the P0300, P0304, P0305 and P0306 codes for the past two weeks. It had the P0170 and P0173 codes and I got a new MAF sensor and those codes went away and now this. When the misfire codes came it was when I was accelerating and from then on I can had to be careful not to press the pedal too hard. Now it shutters and won't go over 5 mph. Any ideas on what it could be?
 
It has 94K on thee odometer.
 
Here's what I've done:
 
CPS replaced
MAF replaced
plug replaced with bosch +2
air filter replaced
 
I am running out of money and stuck.
 
Thanks,
 
P.Creech