Lets say for a moment that your mod is fine and the electronics are working correctly. If you chain vape a RTA with a small vapor chamber capacity (and most are... at least compared to an RDA or genesis atomizer)... that chamber is going to get hot, even if your build surface area, and wattage are relatively mild, and you have adequate or better inlet air flow.
You don't mention air flow settings (wide open, near closed etc etc), your "draw time" or draw frequency... so it's possible your issue stems from not enough air flow, in combination with too long a draw.
Short of a series of instrumented electrical tests... you really can't thoroughly "test" your mod for the accuracy of settings, but... a simple in-line volt meter might provide a very basic sample value, in that a change in output voltage, at the time you sense a temperature increase, would be an indicator that the mod might be defective.
