My sister in laws 04 duramax has been setting a p0087 and surging at idle. She describes it as sitting at a stoplight and it tries to take off on her. She had someone tell her it was the FPR, but I am seeing people talking about fuel lines colapsing, bad injectors, and an occaisional FPR. I am a "retired" auto mechanic but I am not familiar with Duramax diesels. Where should I start with diagnosing this thing?