Pgbouncer/postgres expert - Slow queries

We have a few connections error popping from time to time. it might need some tuning.
We are running multiple instances of pgbouncer (without HAProxy which might be a problem).

Error Example:
- closing because: server conn crashed? (age=59s)
- pooler error: server conn crashed?
- ...

We also have some queries that sometimes take 50ms and sometimes 10sec.
We'd like someone to help us figure out all that stuff.