I have a real old furnace that works ok. the problem I'm having is the air coming out the vents.the air is pushing enough threw the vents to keep my house cold. It seems at night is the only time it feels cool. I have cleaned the coils,oiled the motor,cleaned the blower and tighten the belt.the only thing I can think of is the motor is going bad. its a marathon electric 115 volts,1/3hp 1725 rpms.should I buy a new one with more power?