1988-1991, and 1996-1998. I was born in 1988, and watched regularly until I first saw Placido Flamingo. I ran screaming from the room. That settled it for me. I refused to watch it after that because every time I tuned in, eventually he came on (my typical bad luck)....I have no idea if this is just a colored memory, but it did seem that every time I watched in 1990 or 1991, he came on. That was scary, and I would have no more of it. (However, I did not grow up entirely without SS in my early days-I had a couple of tapes that I knew were OK, including the Put Down the Duckie sketch, "Bedtime Stories and Songs", "Sing Yourself Silly", the tape of big Bird at the hospital, and some episode from 1990 or so with Andrea Martin).
Flash forward to 1996. I was a weird kid with mental illness who spent all his time watching TV. Somehow, I don't remember how, I got hooked on watching (and taping!) Sesame Street every day. That's how it was all through 3rd and 4th grades. Unfortunately, I did not forsee this phase, so I'd already (by this point) erased the recorded (not pre-recorded) tapes I'd watched in toddlerhood, which I kicked myself for doing (I still do). Thus why I've always been looking for that mysterious Andrea Martin episode. However, it was around this time that I found my tape with the partial episodes from 1989 that I've mentioned before, that satisfied me for a while.
Anyway, I bought the Put Down the Duckie VHS in Nov. 1996, not knowing that Placido Flamingo was on it. Once I found out, I got over that fear and watched it every day from then on, until 1998. And let me tell you, I thought those episodes were pretty cool, especially the ones with flashbacks, like 3611, where Leo Birdelli wants to buy Mr. Hooper's picture, and a later one where Big Bird, Gabi, and Maria organize photos, which leads to flashbacks of Maria and Luis's wedding, and Gabi's birth (I taped this one back in the day, but taped over it...I'm still looking for it). Though I liked the newer episodes, what I really wanted to see was the older ones (thus my obsession with the aforementioned two episodes).
But the show seemed to change beyond recognition at about this point, and I was starting middle school, so I finally stopped watching. But I was still interested in older episodes, which is why I was so psyched when we finally got Noggin in early 2002. I taped all the Unpaved episodes I could before they finally took them off the air (and somehow, I lost one of those tapes...UGH! I have bad luck with videotapes).
Sorry for being long winded.