M1 slam fires are somewhat of a mystery, even to the experts.
Here's the little I know about them and what causes them, and I hardly qualify as an expert...
1. Improperly seated primers, primarily on re-loads. A "high" primer, not all the way seated into the primer pocket, will detonate with less contact than it should. Fix?? Properly seated primers, obviously.
2. Firing pin protrusion. If the bolt gets excessively dirty, the firing pin will not "free float" and will make excessive contact with the primer when the bolt "slams home" into battery. Fix?? Clean your bolt, particularly the firing pin, and get the gunk thats making it stick out of there.
3. Improper head space. GET THIS CHECKED / FIXED immediately.
beyond this, I can send you a site (membership required) that all the "experts" hang out at. They are likely to be able to diagnose your problem better than I.