Dave, do you moisten the strap (spittle works better than water) before putting it on? In cold, dry weather my Wahoo strap shows that same pattern of working ok at Z1 heart rates but then spiking ridiculously high when the heart starts to work a bit harder. It seems to be because it takes a while before moisture reaches the surface of my skin from the exertion and so early on in the workout there isn’t enough moisture there to accurately conduct the electrical signal of the HR. My Polar H10 is more accurate (the working part of the strap has a much larger surface area) but I moisten it anyway in dry cold weather because it will also sometimes go haywire.
However, if your strap is giving wacky readings for more than ten minutes or so, this might not be the problem. Have you also tried changing the battery in the monitor? It’ll be interesting to see what results you get with the new strap.