I have a 10-year-old reddit account. When it was static pages served out of lisp, it was rock-solid. Their rewrite got to the instability you're describing, and it never really recovered.
The Python rewrite happened 16 years ago. 10 years ago, there was probably an order of magnitude (or more!) less traffic and this is probably what causes most of their instability.
10 years ago was about the time when they first moved to Cassandra and that was a pretty rough period. Lots of rose tinted glasses around here. Reddit has had plenty of stability and scaling problems during the early years.
The Twitter account named Reddit Status was started in 2010 to track Reddit’s downtime. It has always been unstable. I’ve always thought it was strange how often Reddit was crashing or going down for scheduled maintenance when no other websites ever seemed to do so.