Complex sentence: They were, however, relegated only two years later, and has since then not been able to establish themselves permanently in the Elitserien again, but have mostly played in the second-tier league Allsvenskan, except for a few short stints in the 90s.
Split into simpler sentences: They were, however, relegated only two years later, and since then have not been able to establish themselves permanently in the Elitserien again. Instead, they have mostly played in the second-tier league Allsvenskan, save for a few short stints in the 90s.