I'd the above problems that you stated are true then I think it is pulling a Gox. These are all the telltale signs of an exchange running on fractional reserve and wantinf new deposits to cover pending withdrawals. And this isnt just a problem for the traders, people also take altcoins as collateral for loans and store them on poloniex.

I've been using bitttrex since 2016, never had a problem. Always had faster deposits and withdrawals than polo too.