"wire cage fixtures". If those are what I think they are, they are a danger to your ig. Attracted to the heat, he can find a way to jump up and grab the wire dome, and pull his back legs up, and toast his belly to death. Solid domes can't be grabbed by igs, and they slide off of them. I did lots of experimenting to prove this.
Your fixtures can't do the job if their packages say they can accommodate 250 watts (as long as you're not using long, thin extension cords). The toasted wires are the evidence.
One common thing causing this problem is using long, thin extension cords to run the fixtures. Too long/thin a cord at too high a load causes resistance, which heats up the wire. Lamp wire is a common culprit. I'm convinced you can change out the cord attached to your fixture, replacing it with a number 14 2-wire cord**. Since the wall receptacle where you plug the things into can run a total of at least 1,000 watts without overheating, then a mere 250 watts will be easy for a #14-wires fixture cord to handle.
Is that as clear as mud? (-:
But I've seen a lot of functional but chronically slightly overheated wires that connected to the terminals of the device they ran. Over time those wires just slowly baked, but the damage was within 1 1/2" of the terminals. If the damage crept up the wire further, there is more serious danger of the insulation melting off, letting the two wires touch and short out.
If the insulation's baked appearance on your fixtures' wires extends more than 1/2 inch up the wire, then the cords need to be replaced with ones having thicker wires.
I just found an electric supplies place that has the ceramic fixtures w/8 in. fiberglass insulated pigtails, for less than $4.00 apiece. I used to find them in regular hardware stores, so you can likely find them in various stores and electrical suppliers in your area.
If you can find an appliance repair shop (not the appliance service guy), or a fix-it guy in your area, they probably have this wire on hand to sell by the foot.
**But where the wires connect to the socket, you will probably have about 1/2 to 1 1/2 inch of baked but not shredded or missing, insulation on each wire.
So you have 2 solutions:
1.) Replace cord with a #14 or #12 cord. Expect a little insulation baking near the connection to the socket.
2.) Install 8 inch heat resistant wires (pigtails) between the socket and the cord. They will help dissipate the heat before it gets to the cord. Use ceramic wire nuts, and make good, secure splices.
Personally I would rather do #1.