It's probably not the spring if it is a progressive problem as you describe.
I would highly suggest smoothing the trigger, even doing the setscrew mod listed HERE
READ IT ALL THE WAY TO THE END!!! Some of the first trigger mods didn't last...the best one is the Setscrew Mod.
http://www.rimfirecentral.com/forums/showthread.php?t=143780Also, if you are using dirty ammo like Remington...you WILL have to clean it more often, the crud builds up quick in the action and causes the problem you describe...
So does firing HYPER-Velocity ammo like Velocitors...it makes the plastic Buffer break down...and little chips of plastic can get lodged in the trigger area and cause it to be hard to pull.