Transmission vacuum shifting valve keeps dying

I've got a 81 300D. The transmission vacuum shifting valve went bad & it started shifting hard & going back & forth between gears. I replaced it with a good valve off my 'spare parts' car. The transmission started shifting like perfect again. However, after maybe a month the car is shifting hard again. I suspect something on the car is killing this valve. Note that this model has 3 of the little white vacuum valves on the valve cover. The little plastic levers on them are good, so the 3 valves are switching correctly. The vacuum on the line going to the transmission is 17 when at lower RPMs and 7 when the engine is rev'd. I read somewhere that this should go from 10 to 0 from low to high RPMs. So is this too much vacuum? Could this kill the shifting valve? What else could be wrong & how can I check them? This site & the team have been great. I've had a number of issues on my 3 old 300D/TD's and Kaia has been right on target every time with the right advice, right parts & right price! Thanks -tony