The generally accepted fix for this is an expansion tank, not a TECV. Plain fact, water expands when it is heated. Depending on the system, it can either start to blow up softer hoses like balloons, leak out of the weakest valve, or if your house isn't a closed system, push water back to the water main. More and more water supply companies are installing check valves, which makes your house into a closed system. So, has the water company changed anything recently? Do you have a PRV? What is your static water pressure? How did you test the WH T&P valve to get 150psi? No domestic water system should ever have more than 80psi, and if it does, needs regulation. So, there may be more than one thing going on. But, first, I'd probably buy a water pressure gauge and actually check what you have. Pick one up with a second, 'tattle tale' hand and leave it connected for say 24-hours to measure the peak pressure.
Since most things in your water distribution system cannot expand much, when the water does, it can raise the pressure radically...it doesn't take much expansion. It also doesn't take much to relieve that pressure - a leak somewhere, either intentional or by defect. Better to allow it to expand into the expansion tank, maintaining your set pressure and not stress things.