agm m14 fring problem

[Black Bull]

got a m14 of the swaps thread a few days ago..theres a problem with it when i fire it in full auto it will fire about 5-8 shots then stop dead then i switch to semi and it will fire another few shots then it stops dead again it will not fire whatsoever and then it becomes very tempermental and repeating the following process i have tried this with a number of batteries so i am thinking this may be a connection problem...any thoughts?