Head unit goes off above volume 25, then comes back on?

Fitted jbl 6X9s to rear as my golf mk4 had none in the back. Running speakers normally off of block from rear of head unit. checked/replaced the wire today going from the head unit to the speakers. Still shuts down over 25 and comes back on. I think the radio is 52w x 4. Its a Sony MEX BT2500. No am or sub, very standard setup