You have a continuity issue, likely with your mech switch assembly. When the mod heats up, particularly in the switch assembly, it's a indication that you have a substantial resistance, causing a "
Joule heating" condition.
If you've not disassembled and cleaned the switch assembly... do it. Remove all tarnish and/or corrosion (if any) using what ever methods you have available. A pencil or pen eraser works nicely for removing tarnish from copper and brass contacts.
Check that your negative button contact extends above the switch battery resting shoulder when the button is pressed and that there's no battery rattle when the button is not engaged.
If you have a DMM, you can test the mod for resistance/conductivity... should be zero Ω. If you have an inline volt meter, you can test for voltage drop... another indicator or conductivity.
If you can't sort it out, or it seems uncorrectable, I would suggest raising the atomizer resistance a few tenths of an ohm at a time, until the switch cools down.