Supposedly one engine either gave out or had problems.

Bird impact.

It never came close to specifications of climb-rate/acceleration, service ceiling and range to name the three most important ones for an interceptor.

Do you have more details? most publications emphasise that MiG-23P (its partial replacement) had a radar with look down-shoot down capability compared to Su-15, but do no mention performance. Its certainly strange, as it would make more sense to install the radar in the Su-15.