If you ran out and picked up Pirates third baseman Pedro Alvarez after his six-game run last week, in which he hit .368 with four home runs -- well, the joke's on you. Alvarez is one of the more unpredictable players in MLB, and picking him up after he does something like this is just going to cause headaches.
Alvarez hit 30 home runs last year, with eight of those coming in multi-homer games (two of those games coming on back-to-back days), two homers coming in a doubleheader, and four homers coming in a one-week span in July. Alvarez doesn't hit homers in streaks -- he hits them in bunches. By the time you notice he's hit three or four and pick him up, he puts that big bat down and gets back to unspooling all the good Fantasy karma he just accrued.
Alvarez is a decent option for Roto leagues -- over the course of a year he will get the home runs. But trying to work him into a lineup in weekly Head-to-Head formats is maddening, and should be avoided at all costs. If you're starting him, and your team is, say, 3-1, consider yourself a very lucky man.