Since a while I noticed the ram on my raspberry gets full over time caused by a system process. I think this could also cause the issues you described. Have you already checked the memory usage?
Not much can be done about RAM resources on a RPi other than allocating the min. possible to the GPU which I have always done, RPi ZW's having a total RAM of 512 Mb. I have allocated 64 Mb. to GPU on all my RPi's which run DSLStats..
My RPi ZW's sample every 120 secs. and are configured to upload snapshots only to a web server every 2 days, did try to run FTP data uploads to the web server but the RPi ZW's could not handle this, CPU max'ed out.
FTP data uploading to web site via DSLStats works fine on my local RPi 3B as would be expected.
I don't think RAM resources are the root cause of my current issue because:
1/. The same setup worked perfectly for over a year without any remote logon issues.
2/. The exact same setup is still working on my other remote RPi ZW without any logon issues.
Just remembered, one difference does exist between the two remote RPi ZW installations, a Sam Knows White Box is in service at the installation where the remote connection problem exists, always has been the case, it branches off a router LAN port then feeds a desktop PC and smart TV via wired ethernet connections, can't honestly see how this could effect the cloud connection to the RPi which has a direct Wi-Fi connection to the router ?