The Cubs had somewhat random playoff runs in 1984, 1989, and 1998. Each was bookended by losing seasons. In fact, from 1973-1992, the Cubs had only 2 seasons above .500, and both were division championship seasons.
The Cubs were more successful in the '00s, actually managing 6 seasons above .500 with 3 division championships, including only the second consecutive postseason appearances in Chicago's MLB history (last year was the third; the White Sox never have done it). The randomness was on the other end, as they finished in last place in 3 of their 4 losing seasons.
But it wasn't until they hired Theo Epstein that the Cubs had both a commitment to consistent winning and the management in place to pull it off. ND hasn't had that since Ted and Ned retired, although the signs of decay didn't become apparent until the 1994 fall-off and alarmingly obvious in 1997 and beyond. Now it seems all ND can hope for is a random great season like 2012, or even a random good one like 2015. We've had only two top 15 seasons in 20 years, and only one run of consecutive elite bowl appearances (2005-2006, both losses of course).