I found this source. As I recall somewhere in beween TOS and TNG.
That episode is a good one. There are so many good ones. I place TNG writing at the top of the ST universe.
Definitely have a look at Yesterday's Enterprise. That helped to solidify Human/Klingon relations, just as the link above mentioned. The Khitomer Accords helped as well.
So now I am looking for evidence of warmer relations between the Federation and the Klingon Empire prior to 1988. I didn't see any of that in TOS and don't recall anything in season 1 or the first half of season 2 of TNG.
As far as finding something between 1998 and 1991, it will be hard, because keep in mind that TNG started roughly 75 years after TOS went off into the sunset. IIRC, Season 2 actually started in mid/late 1989, and Season 3 was roughly the same time in 1990. Yesterday's Enterprise was in TNG Season 3, so finding anything during or prior to 1988/1989 will be hard to nearly impossible.
BL.