What is the ability that sylar stole? to clone into other people right?. so that means he touched A LOT of people, if you notice all the people that sylar had touched since he got that powers are the people that he transformed into.

now the real question is WHY SYLAR TURNED INTO ALL THOSE PEOPLE: well folks, the reason he turned into all those people is again, his agenda was to steal the presidents dna, so that meant he had to touch the president. BUT because he was actually touching peters Petrellis he ended up only “stealing” back his own dna and when he tried to change into the president (kkind of like a HA HA i am sylar and i can do anything so i'll turn into you and then kill you!) he ended up re absorbing all of those people, hence the changing back and forth to all of them. because if you notice, the people sylar turned into, were the people he touched… right?

And as i am sure, but nt confirmed every persons ability has an on off switch. and i'm sure that the reason he also changed is because he had to touch him and “absorb” the dna at the same time which was BOOM immediate overload (it's like loading the same file on your comp 10 times, doesn't do any good but take up space and memory, and he ain't a computer)

so to recap

sylar tried to absorb the presidents dna, it was really peter
and because of that he ended up absorbing all the old people he had previously absorbed and it overloaded his “senses” and made him sporatically change. Peter did not because peter was in control he KNEW what was going on so he was lets say, mentally ready for it. And he never took sylars abilities away permanently. i would say he just stunned sylar with the good lovin' aka butt whoopin and good ending.