audio issues with pioneer 5800 hu

I am going nuts trying to figure this one out. I have a 91 Mazda Protege. I have a Pioneer DEHP5800 head unit and all pioneer 5 1/4 all with base blockers. I have a MTX thunder series amp and 2 lanzar 12 inch 500 watt subs. Issue is that I get audio cuts at a little above half and gets more frequent the higher you go. But this is the kicker, it only does it while the car is running. If the key is in run position and engine off it sounds great. If in run position with the engine running it cuts the audio to all speakers. I have not seen any dimming of the lights and dvom shows 14.2 to 14.5 volts regardless of the sound from the radio. I checked grounds at radio and amp. I have .7 at hu and 1.0 resistance at amp. I have put a noise cap on the power feed to the coil and still there. If i turn the hu down it almost goes away completely. I have had it to a highly recomended shop and paid them 85.00 to tell me they do not see anything wrong after they said it was hu and warrentied it for a new and still have same issue. Any info would be greatly appreciated. Thanks

How is the HU being powered?  I suggest trying a new (fused) power connection directly to the battery and a new dedicated ground direct to the chassis for the HU.Support the12volt.com

Did that. I have 4 guage ground from battery to amp direct also have fused power from battery and switched power from the ignition slot in the fuse box. I did change the rca cables to the amp and it got better but still there. Although it did it probably 100 times before and 10 now. I have been installing for years and never had this issue before. any other suggestions would be great and I will recheck the power and ground to the radio from battery.