Sounds like the Battery/Mod itself is just dying, Your tanks coil wouldn't impact a Regulated Mod with Internal Battery like that I dont believe. Most new mods come with TC options but still have regular Wattage mode for Kanthal Coils. Look into Sigelei 150W dual 18650 setup and Sigelei 75W or IPV D2 for single 18650 usage. As you search around for those, you will likely start to notice other mods like them

When you find one you like, feel free to post or PM me any questions about What Batteries to use in them and what Chargers to look at!