1989 Olds 98 passenger-side speakers

The speakers on the passenger side were all making crackling noises every so often....would work fine for 40 minutes, then start crackling, then work fine.

I have no idea why it's still doing this. I replaced all the speakers. The head unit is stock...15 yrs old. If it were the head unit causing the probem would it work fine....then crackle...then work fine? Ideas?