The CT-F700 was a pretty decent deck, especially by the standards of the day. The first obvious solution would be a full cleaning of the heads, capstan and pinch roller. Second would be an azimuth adjustment, which, while you technically need a test tape, I have done it by ear with a known well-recorded tape. And after that, it might be that the electolytic capacitors in the deck have started failing. HiFi engine has a
full set of service manuals for the CT-F700, which are quite comprehensive. If you don't have access, PM me your email address and I can send them to you.