My cat did this because she had a UTI and bladder stones. The doctor told us "that's just what cats do. You must not have owned many cats." Which was the most annoying thing I'd ever heard. I would check for UTI or even stones (with an ultra sound specialist, not just some nurse/vet at your local vet clinic). Of course, this could be something entirely different since your cat is quite old.

My cat ended up being diagnosed with anxiety & FIC which means giving her generic Prozac (liquid fluoxetine) every night. Its been a year or more and we haven't had any issues.

I'm really sorry this is happening to you. I know first hand how stressful this is and I hope that things get better for you and your cat.