7-cylinder stumble

Desperately seeking a clue: 1997 Crown P71, 95,000 miles, former State Police car, excellent condition EXCEPT: at idle, or low rpm under load (such as in O/D at 35 mph or less), the car has a miss and runs on 7 cylinders. I know it's the #1 cyl after checking code & scanning from Check Engine Light (flashes 20-30 secs then goes solid). I've replaced plug, wire, coil has spark with wire off when running, replaced injector yesterday with no difference, took new injector out to put old one back on & it was full of fuel so i know fuel is getting to it under higher rpm, car runs on all 8 at 37-40 mph+, drove with #1 injector unplugged and it ran rough at ALL rpm so I know its clear at higher rpm. Confused! Could it be coil, or TPS? Suggestions on testing/checking either besides a visual check for spark on the coil?

still searching for answers... have replaced coil, injector, all 8 plug wires, and today, fuel filter. STILL has the at-idle miss. Even talked to the chief tech at State Police garage, he said it sounds like a fuel starvation issue; checking fuel pressure tomorrow. Anyone, ideas???

check plugs themselves.