Slow to no internet after a while with Mojave

I have one imac running mojave and one running high sierra. On the one with mojave, after a while safari will not connect or download some site, but it does others. Apple mail does not update and mail messages come with no content or stays on downloading content. After a restart everything works for a while and then at some point back to no internet. I am using an ethernet connection. The imac on high sierra does not have any trouble connecting to the sites that the mojave imac can't connect. both use the same cable modem.

Any suggestions?