I remember three episodes, but there were probably more:

1. "A Perfect Memory" - probably the best of the three, a second season episode, on MeTV today.

2. A sixth season episode, this one is with a woman he can't remember, and she calls him Stevie (which would be a deal-breaker for me!). Silly episode.

3. A tenth season episode, with a woman named Sylvia (Jane Wyman). She's a woman they all knew well, from Bryant Park, who somehow was able to find Steve in Calif. But now we have Barbara to get jealous. She was never in any other episode, which makes this premise silly, also.

And of course, we had to have a Barbara old flame episode (he called her "Bobbie"), in the eleventh season. IIRC, perfect Steve didn't get jealous, until the end.

I liked Charley's comment regarding Steve's "little black book" to the effect that if the ink has faded, the woman is now too old...!

The main one you'll be able to add to your list: "Fly Away Home" - Season 7, ep. 2 (Fall 1966). Steve goes back to his childhood home and has dinner at the home of his old girlfriend (played by Virginia Grey), now married. She and Steve had been in a serious relationship. Lady with a bratty daughter and a husband who's not to thrilled with the idea of hosting Steve.

In that one we see the "Mayberry" sets from TAGS. I like the episode, the women who guested (Ms. Grey and Ann McCrea) were very good and likeable. Funny comments from the husband, "Well what do you know, Douglas is a freelance!" The crowded dinner scene is memorable, if not somewhat disturbing...!

Yes, I remember that episode now.

I see from the TV Guide there's another one coming up, from season 2. He sure got around, and he never ever married any of these women (I assume)!