With older cell phones I've had similar issues I'm sure you may have tried this but in case you haven't I've found gently cleaning the contact points on the actual sim card with a pencil eraser will improve electrical contact since the tiny metallic points that hold it in place may be wearing down the actual sim card which probably has become slightly contaminated with dirt and oils from ungloved handling while removing and inserting the item into your phone its free to try and couldn't hurt just be careful not to break the fragile gold color sim contacts. Good luck!