Help - p0305

I have a 1995 Mustang 3.8L with a single code...P0305. I verified spark and fuel. I swapped plugs, wire and injector from #5 to others. Still get P0305. I can hear injector clicking away with a stethoscope. Checked compression. 142 on #5 ...lowest compression on others is 140 with highest 150. I did a engine block test. It was negative. Need help with what to do next.:banghead:

Thanks