I know you stated you have all new brake hoses, but I think I would replace the rear brake hose again. These days, being a new part no longer means its a good part. As far as the brake hose is concerned, you really have no idea how many years it has been sitting on a shelf waiting to be installed, or what may have been sitting across it during that time.

My bet is either on the brake hose, or the calipers. If either has a delay in releasing, that delay could create a lot of heat pretty fast in city driving where you use the brakes a lot in a short time span.