I recently picked up a whisper myself. Out of the pellets I had on hand, mine shoots Beeman Field and Target round nose the best. RSW Superdomes were a close second, then JSB Diabolo exacts. Crosman highpoints shot like crap. Beeman CroMags did ok. And don't forget to clean the barrel as mentioned in a previous post! Also, if I'm not mistaken, the scope that comes with the gun is set for 100yd parallax, not a whole lot of good for the pellet gunnin'. For my $45 the Leapers 3-9x32 Golden Image is worth every penny. After Christmas I think I'll chuck the Gamo scope and buy another one.
Try shooting the gun without the scope for awhile! I have better groups with iron sites than I do with the gamo scope. Also, when you mounted the scope did you make sure to clean the rail on the gun and the scope mount, and did you use locktite on the mounting screws?? Scope shift might be part of your problem too.