Try this,
Remove the base and screws and clean everything with Gun Scrubber and a stiff brush for the screws and, if you can find it, a little wire brush for the tapped holes. If nothing else, get a tap and using plenty of Gun Scrubber, run the tap through the holes. Blow everything out with some canned air. Now, use a little MEK or lacquer thinner (nail polish remover) treat the threads again and blow off the excess. Let it all dry for a bit. While it is drying, see if you can find new screws with Torx heads.
Someone else will have to give you the screw sizes, I don't know them.
With everything immaculate, install the base again with one small drop of blue Loctite on each screw. Torque to specification. Now put everything away for 48 hours. No cheating.
Go out and shoot again. If things work loose yet again, I would go to a gunsmith and have him enlarge the holes and tap them again using the proper drill size for a tight fit on the new screws. Go through the cleaning and Loctite deal again and you will probably be good to go.
Have a look at the base to action fit as well. For the screws to lock the base in place, the curve of the base needs to make full contact with the body of the action. The less base in contact with the action, the less clamp force you will have and the recoil of that WinMag with a heavy scope on top will be like hitting the base with a hammer each time the gun fires.
Another point. Make sure the screws are in full engagement with the action. You want that screw flush with the inside of the action, even if the gunsmith has to grind any protrusion off to get the bolt to go in.
