recursive method not reaching base case

[possibilitybox]

i was working on implementing the original supermemo algorithm (see
http://www.supermemo.com/english/ol/sm2.htm for a description of it) in
a class, and i'd just finished up the first draft.  it works for
repetitions one and two, but on repetition three (you must manually
increment item.reps.) or higher it recurses until it reaches the limit.
 can someone point out what i'm doing wrong here?

here's the code:
class item:
	def __init__(self, key, value):
		self.key = key
		self.value = value
		self.reps = 1
		self.ef = 2.5
	def interval(self):
		if(self.reps==1):
			return 2
		if(self.reps==2):
			return 6
		return (self.interval() - 1) * self.ef