Valve adjustment oddity, or not?

[VegeMan]

In adjusting my valves on my '81 300D nonturbo I am perplexed with an odd occurence. After adjusting a valve to the right gap and tightening the lock nut and adjusting nut, I check the gap again and all is good. But when I use a wrench to spin the valve stem using the two nuts, which turns the valve stem and the spring assembely, the gap changes, even though the nuts are tight. The gap will get either tighter or looser, but if I keep spinning it it will return to my original adjustment. That is, if it gets tighter, by spinning it further it will eventually loosen up. Is this the sign of bent valve stems or a need to overhaul the head? If not, then is there a trick to adjusting the valves that will eliminate this occurence? It seems that adjusting the valves correctly will only result in a change in the gap as the valves rotate when the engine is running?
Thanks for any help,
Rusty