Yup, alpinegreenneon is right. The tips are melting because they are getting too hot. First, make sure you have the right heat range plugs for your car. I've seen plugs with the wrong heat range get mixed up with those that are right. Get a new set and check the numbers.
As alpinegreenneon notes, it could be predetonation too. Are you getting any engine pinging when you drive up hills? Are you using a regular fuel and should be using a higher octane?