[nom-dun-chien]

Lenny, our 11 month old Yorkie, has taken to biting in the past couple of months - passers-by, cyclists, people in the street who try to pet him. He did actually bite my husband one night when he arrived home on his bike - Lenny didn't even bother to see who it was, he just lunged and nipped. But otherwise he never bites us or the kids (aged 12 and 7). In an open space, such as a park, he might charge 150 yards across the grass to have a go at a couple who are just walking along minding their own business. It has happened about seven times and now we daren't let him off his lead if people are around. Any suggestions as what to do? We have another dog, a 4 year old male, Cookie, and he and Lenny get along fine. Cookie does not bite although he barks a lot when people walk past our house. The two dogs have human company most of the day since either myself or my husband are around much of the time.
Any helpful advice would be greatly appreciated. Thanks, Alison