It is a problem with the safety, it needs to be disassembled, cleaned and lubricated and reassembled. After removing the firing pin with the shroud from the bolt body the firing pin, the safety, safety plunger, spring and stop pin need to be removed from the shroud. Clean all parts and use a high quality gun grease when assembling.
There is a schematic at the following link
http://stevespages.com/ipb-winchester-70.html early model
http://www.gunpartscorp.com/Manufacturers/Winchester-33573/Rifles-42602/70Post64ControlRoundFeed-43622/PartsList-43631.htm?results=All late model