Not getting GiG speed I have tried everything

The fastest speeds I have seen my speeds on download s be is 900 mbs. That only happened once or twice on average my speed test from Ocala will be in the 700s MB’s. The upload speed seems fine I do get up to 40 mb upload . My modem downstream and upstream signal seem within acceptable levels . I m not sure if this has anything to do with the issue but from time to time through out the day my upload speed will go to 0 MB’s . Please anyone .