WEIRD SURGING IDLE

Hi Shocker. Your Mustang's problem sounds similar to a problem my 2006 GT had several years ago, so I thought I'd share in case it helps you. When my Mustang was a few years old, it started surging randomly. Always under 20mph and eventually when in park. It would do this when driving through parking lots, slow city driving, and when slowing for a light or stop sign. All this happened after the Mustang, over it's first few years, became gradually very slow to respond to the accelerator, then would peel out when the fuel hit the engine. It NEVER tripped a dashboard light and my trusted mechanic never found a problem during inspections, even when he was able to reproduce it. The problem was finally diagnosed after several years of this (and grew alarmingly worse - the Mustang would strongly surge forward while the brakes were applied hard), when the car stalled twice while driving down a (thankfully straight) steep hill. We immediately took it back to the mechanic, who told us to take it to a Ford dealer we trusted. It turns out the gas line from the tank to the engine was being compressed and worsening with time. That's why the acceleration response was slow, then it would surge when the fuel got to the engine. It was a $400 fix, but it has never done it since and that was maybe 4 or 5 years ago now. I hope this helps you!