Hi! Generally, "a lot of content" is a bit subjective, but optimized emails tend to be on the shorter side (depending on the purpose of the email, "short" is also relative). But if you usually copy from an old campaign. make changes to the existing content and copy/paste in new copy, and the email is longer than about a standard page, it's possible something is getting mucked up in the coding. But even this is variable and other users have not necessarily experienced the same problems.
Out of curiosity, to continue the troubleshooting, have you tried creating a brand new, fresh template and played around in it to see if the same behaviors occur? Even if just for the purpose of this investigation, it would be helpful to know if the issues persist. Likewise, if you work in one of your old emails in a completely different browser or in Safari's safe browsing/incognito mode. Sometimes there are random extensions or old cookies/cache that might be interrupting the experience, as well.
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