what causes sibilance?

Clearly, you may receive many different answers/opinions on what your problem may be. So my opinion will only be one of many!

While the Disc Doctor Miracle is supposed to be a top quality cleaning product, it is also, from what I understand, notorious for needing LOTS & LOTS of rinsing. I am only venturing a guess here that perhaps you did not rinse enough on that particular record and that residue from the Disc Doctor may be contributing to the sibilance you are hearing. If you did not have sibilance issues before, a re-clean of that record followed by heavy rinsing, perhaps 2 or 3 times with high purity water would be the first thing I would do to see if that solves the problem.

only on that recording? If that is the case, I'd say it's the recording that is sibilant.
If you have the cd version, it would be interesting to compare as far as sibilance, although it may not mean much in the end since the record is dated to 1978...obviously CD will be mastered differently and much later, but still an interesting test. If it's not the recording, reclean with other fluid may be...and if that doesn't help, check the VTA.

Lots of reasons for sibilance. It's just that some systems will be more prone to it than others. Depends on the overall balance of the system, components themselves, type of tweeters(metal domes are more prone to sibilance than say silk), etc...

thanks for the reponses. will try vta adjustment and re-rinsing. The cd version doesn't have any sibilance to speak of. It's probably not just this record, I only recently started cleaning them adequately.

Don't forget to keep the stylus clean. That was my first experience with spitty sibilance 35 years ago--a big ball of fuzz stuck to the needle. Use a Mr. Clean Magic Eraser, and then a soft brush to remove the Magic Eraser residue.