I'll give you both right here, hehe. Duke has a point in that Sauron's ability to wield his power at will apperently was restricted to some degreecompaired to his capacity before Númenór sank , and Øctavia that Sauron with the Ring Sauron was stronger than before or after [thus during the period 1600-3441 Second Age].
But actually he was only artifically weaker without the Ring than with it. By which I mean that if I get in a car I can move much faster than on my legs, but when I step out of the car I can not move less fast than normally on my legs, I just can't move as fast as in a car that I got use too and other think of me being able to
However that Isildur took the Ring was not a matter of luck, as Sauron had already been defeated and was all but killed, Isildur just walked over and cut the Ring of the unopposing foe [as I read it: least we have no backing for the PJ version of it to be clear], and then the spirit left the incarnation fully.
EDIT: Some lore that may be fun: