Ughhhh....Giardia. I know what you are going through!!
Ralphie just got over it, he had it for about 4 months. It can be very tricky to get rid of. He was on 4 different medications for it (Panacur, Metronidazole, Albendazole, and Ponazuril.) Ponazuril finally did the trick. Not a lot of Vets know about this medication yet from what I am told.
Also, it is extremely important to keep his area clean. Bleach will kill the cysts. His drinking and eating bowls should be cleaned after every use and bleach should be used to clean up the floor if he poopies there.
Hang in there. Maybe you will be lucky and it will only take 1 medication to clear it up. Let me know if I can help. After 4 months of the stuff, I kind of feel like a Giardia/poopie expert