It sounds like you've hit the postgres basics, what about some of the linux check list items?
what does free -m show on your db server? If the load problem really is being caused by swapping when things really shouldn't be swapping, it could be a matter of adjusting your swappiness - what does cat /proc/sys/vm/swappiness show on your server? There are other linux memory management things that can cause postgres and the server running it to throw fits like THP and zone reclaim. I don't have enough info about your system to say they are the cause either, but check out the many postings here and other places on the detrimental effect that those settings *can* have. That would at least give you another angle to investigate.
