Netgear R6220 upload performance decline after a while for host in DMZ

I have the R6220 as a main router at remote site connected to my ISP's fiber(1000/1000mbps connection. I then have a downstream pfsense firewall connected to it. The pfsense box has a static LAN ip which is set in the DMZ of the netgear router. Essentially all traffic from netgear gets forwarded to pfsense.

I can access several of my computers that are behind the pfsense box. via different protocols, sftp, webdav, SMB etc.

Every other day or so, the upload speed from every machine behind the netgear router comes to a crawl. The upload speeds for file transfers e.g using webdav seems to get stuck at around 200KB/s(Note its a gigabit upload ISP there). I' ve done extensive troubleshooting, replaced all cables and switches and this issue reoccurs.

The ONLY thing that fixes it temporarily is a reboot of the netgear router. After the reboot i start getting my usual 200-300mbits upload to me(im 1500 miles from that remote site, so this is good enought speed) There is no QOS or any such feature enabled on the router. Its on latest FW. Is there perhaps any bug in DMZ traffic forwarding, I'm just clueless at this point.