Word to describe a relationship that fluctuates between being good and bad

frenemy

noun

Informal. a person or group that is friendly toward another because the relationship brings benefits, but harbors feelings of resentment or rivalry: Clearly, turning the competition into frenemies is good for your business.

source https://www.dictionary.com/browse/frenemy

"an unstable relationship"

• "liable to change or alteration, an unstable economy, unstable weather." MW
• "disposed to temperamental, emotional, or psychological variability." TFD

You could also use "unsteady" or "shaky".

Hot and cold.

https://www.youtube.com/watch?v=kTHNpusq654&ab_channel=KatyPerryVEVO

'Cause you're hot then you're cold You're yes then you're no You're in then you're out You're up then you're down You're wrong when it's right It's black and it's white We fight, we break up We kiss, we make up (You) You don't really want to stay, no (You) But you don't really want to go-o You're hot then you're cold You're yes then you're no You're in then you're out You're up then you're down

While I'm not sure if this qualifies as a "single word", I can't believe nobody has suggested love-hate relationship yet, which has all sorts of rich metaphorical usages beyond sexual relationships.

Wiki also links to on-again, off-again relationship, which is typically but not exclusively used in a romantic context.

And for Japanophiles, tsundere describes a similar dynamic/perennial trope of teen romance stories, which has been enjoying a steady rise in usage since the turn of the millennium.

Such a relationship can be termed bittersweet.

M-W:

bittersweet adjective
Definition of bittersweet (Entry 2 of 2)

1: being at once bitter and sweet
especially : pleasant but including or marked by elements of suffering or regret
a bittersweet ballad
bittersweet memories