Mags release easily when not under tension.. eg. easy to drop and reload when empty. the amount of force on the mag notch is based on the tension of the spring under load. You idea of putting a new spring in the mag, is where I would go... first.
Bill aka ET
Its either the mag spring, or the release spring, I'm betting on the mag spring.
There is also a slight chance the mag lock notch has been rounded off over time.. a simple visual and a file can correct a rounded mag lock notch.
If you purchased this new, chances of a bad lock notch are negligible.. not enough cycle... also if bought new chances the mag spring is weak is also negligible ( not enough cycle ).. which would lead to a defective mag release.