I had a mod that did that (wild resistance changes). Bad connection on the 510 or the ground. Wasn't the tank. Had to take the mod apart and re-solder the connections, then it worked great. It was a Kanger Subtank 50 watt mod. Some of the RTA build decks are so touchy, if the coil is to high, when you put the top on it shorts out, or a tiny little piece of wire hits the sides.
Build as short and close to the air hole (I don't have your mod) as possible, but it can't touch anything. I use subtank minis and TFV4 exclusively, all with the RBA deck. Coils never burn out for me, they get gunked up. I vape a lot of juice (25-30 ml a day), so I can get about a week out of a coil before new coil or just burn and rewick. That's using 2 mods, so 3-4 days a coil. (You can laugh now)
Also, you have to really tighten those little screws down to hold the wire. What I do is put coil in, tighten, cut wires then heat it up to a very dark red (not that hot). Blow on it to cool it. Tighten screws again. Check resistance. Put cap on, check resistance. Should barely change. Fill up juice, check resistance, hit it.
I make the same coil every time, 24 g 7 wraps 3.5mm diameter ~.8 ohms. I can usually tell if something is wrong pretty quickly looking at resistance. I vape 18-25 watts, anything more is too much for that setup. If I need more power, I do the same thing with twisted 26g wire. Then 40 watts is fine.
0.8 ohms is still sub ohm. And it's a good vape...