Did you install any custom firmware yet? Would help to see both the set and live resistance values.
On AF, my iHeat is detected at around 0.36ohm when it starts, with the live resistance value anywhere between 0.34 and 0.38, depending on the mood of the 510 port (yeah the thing has a life of its own)
Power is set to 28W, set temperature at 140°C. Coil material TFR316L. PI regulator enabled (900, 80, probably not optimal but good enough for my draw speed) No pre-heat.
The times I had the exact behaviour you describe was when my brand new evic Primo mini went haywire (and never recovered unfortunately) After investigation it was a problem with the 510 and/or the iHeat 510 port.
Is your iHeat wobbly on the 510 threads when it's not fully screwed? Mine is, on all my mods. As if the screw flanges (don't know if that's the correct term) are not wide enough. On my evic VTC mini it's not really a problem: if I screw it fully I got the correct resistance (or around it, as said above, but I can compensate by changing the set temperature)
On the Primo mini, when it went bad, with the iHeat fully screwed the resistance jumped to above 0.4! With that high of a value, the mod believed that the coil was above 200°C when cold!!! And indeed when I pressed the trigger, it entered protection immediately as it was already above the set point. To make it work I had to unscrew the 510 slightly, resistance dropped to a more realistic value (btw I triple checked with high quality ohm-meters and I know that mine is 0.363ohm, so when the mod displays something else I know for sure it's off)
The problem is that with the wobbly 510, it became unusable partly unscrewed: there was some lateral play on the threads, and just drawing on the iHeat made it move ever slightly, and this in turn altered the live resistance while I was using it, making it completely unpredictable.
With the VTC mini, a variation of 0.36 going to 0.38 can mean a computed temperature variation of over 40°C! And if it goes the other direction towards 0.34 it's the same. In one case the vape is anemic until I bump 40°C up. In the other case the vape is too hot and can char in one hit, until I lower 40°C. You see this is pretty wild and why I say TC is super finicky on my Joeyetech devices. Everytime I start I need to double check that it's not tripping with the resistance value.
These days I found a position on the 510 that is relatively stable. I don't touch it at all and it only varies between 0.35 and 0.37. When it happens I nudge it gently and if I'm lucky it gets back closer to 0.36. But it's never precise.
I don't have these problems with the SmoAnt Knight v2, but it doesn't fit the iHeat being a SBS design. So I'm not able to determine yet if it's a problem with the 510 port on the Joyetech devices (the trifecta with Wismec and Eleaf are all 3 known for having crappy 510's) or if it's a problem with the 510 threads on the iHeat itself, in which case it would be wise to try to source a better base / deck.