Use Windows 7 - MS tells me I cannot update to Windows 10 due to the way HP made my computer - too bad!

My emails come in via att.net and are forwarded to my Windows Live Mail account. 

From time to time, cannot pinpoint the particulars, I can be typing in an email - going or coming - and my type is delayed (quite a while) in appearing on the screen.  And if I try to delete just a word or phrase, the entire body of the email disappears.  What is causing these glitches and how can I stop them?  A friend in another state also has this same problem intermittently.  Just poor Yahoo serivce?