sounds like the 2 years of having limited power going to your speaker has burned the voicecoil, what happens is when a speaker is in need of more power than available then the supplying amplifier (in your case the radio) overworks and sends a clipped signal to the speaker (distortion basically) this distortion is usually associated with dc voltage which creates excess heat and burns the voicecoil.
your's is heating up to the point where once it gets to temp the voicecoil is losing contact internally.......
i would replace the speakers with either a more efficiant speaker (less power) or add an amplifier
