20yrs old, first gun replacement....replace the liner. Not real expensive and it most likely needs it. Most big shops that use MIG will need to replace a liner every 6 months or so. The service manager we use at the plant ( he is Miller trained) has several shops that they service that are replacing liners at this rate. Saves in the long run with down time.
You may also think about wiping the wire prior to entering the drive rolls. I use a cigarette filter on mine, both at home and work. Cheap, and the fibers in the filter will clean the wire and leave no residue. Learned that little trick 30someoddyearsago
If you need it you can purchase a small can of wire lubricant and place a couple of drops on the cleaner, this helps tremendously with the feeding of wire. Check you local welding supply for this, IIRC, it is about $3-5.00 a can. Will last a long time.
Check your tip. A good cleaning with tip cleaners may be needed. If the tip cleaner doesn't do the trick then replace the tip. Often a small piece of splatter will 'weld' itself to the end and cause problems. I have (in extreme cases) filed the end of a tip and cleaned the orfice with a tip cleaner due to this. (best if you can replace the tip tho.)
If the gun is still getting too hot you may need to replace the entire lead. It's kinda like if your stinger gets too hot in your hand you can check the ground and/or stinger lead for a bad connection repair/replace as needed and presto changeo the stinger no longer gets hot. The same with the gun.
Sorry about the length but hopefully something here will help. Good luck.