I know I want it to work so bad. It has such a clean flavor when they start out it amazes me but just can't get it to keep working like that. When it starts out all you taste is the juice the way it's supposed to taste. I've got too many rolls of NI200 to give up completely.
I, like you, was frustrated and wanted to give up on TC.
I got some of Stealth Vapes tempered 28 gauge Ni200. But the key was I finally put that coil in a cheap clone RDA, that has good solid connection points and NO adjustable center pin. The main word in TC builds is SOLID. All wire connection points and contact points must be solid. There are lots of RBAs, RTAs, & RDAs that do not have SOLID wire connection points and SOLID 510 connection points.
Once I stumbled upon trying this RDA I finally understood and appreciated TC. I have two DNA40 devices and I can put this RDA on either one and even move it between the two. It doesn't jump out of TC mode and the ohms don't fluctuate more than .01. It just works.
When you find something that holds the Ni200 coil SOLID and have SOLID 510 connections are SOLID, then it should work. Also make sure your firing pins and 510 connections are clean.
I haven't tried any pre-made coils yet so I can't speak to them other than saying make sure the coil is screwed in VERY TIGHT and all contact points are clean and SOLID. One thing I saw posted somewhere was that Evolv had been quoted as saying the atomizer needed to be crank down on the mod which is counter to everything I ever knew about vaping. But I have a feeling it is necessary with TC to achieve those SOLID connections.
Good luck, don't give up.