Email Sync Problems

[Mimzwart]

I've had my Galaxy S4 for 2 weeks now. Suddenly yesterday I started having problems with my yahoo email not syncing properly. It is telling me that the login failed. If I turn the phone off, it will sync, but when it's up and I try to manually refresh, it gives an error message. I spoke with Samsung tech support last night and while trying to figure it out, it suddenly started working again, but a few minutes later and since then it won't sync. I've checked sync schedule, passwords. Anyone else having this problem?:-\