Heat then no heat

[Rblaney21]

Posted on its on thread yesterday but not much response. My 99 expy has heat for a little while then on longer driving about twenty minutes it starts loosing heat. It has a new front heater core,new tstat,new top hose. The rear doesn't heat and needs a coil. Could it be the water pump or air pocket or real coil causing issues. The outlet to front coil isn't hot like the inlet when heat goes away but the lines on back coil are both hot. It's crazy