You may have found the problem. It may not be an original mainspring, but a replacement with a spring that was too long sometime during it's life. You can buy replacement mainsprings in three different weights individually or as a set of three from
www.gunsprings.com If you buy the set then you can vary the adjustment based on loads. there is an excellent article in the Technical information pages by THOR on the procedure for changing mainsprings... not a task for the feint of heart, but not really that hard.
You can cut one coil at a time from the spring that you have and test fire it with your favorite load until it passes muster.
Good Luck and let us know how thing turn out.