High amp start up draw

Question

I have a Craftsman 2.5hp radial arm saw. When trying to use it on a 15amp circuit (as the manual says) it doesn't come up the full speed and trips the circuit after running about 20 seconds. In checking the draw on the line I found the motor pulling 39 amps on start up and I show 123+ volts on the line. If I plug into a 20 amp circuit it comes up to full speed in about 5 seconds drawing 39 amps then dropping to 9 amps when reaching full speed, line is showing 125 volts Thoughts as to why?