Giving it a second thought, I'd suspect a hardware fault. That's because it 1) writes "Error" 2) goes unstable only at high temperatures where, I suspect, it needs to supply more power.
If you can't get a replacement, I'd suggest inspect the controller board. May be there is a decoupling capacitor missing, or loose wire. May be power supply is not capable of providing enough current, or protection trips, or something.