I'm getting no where with this and definitely need help. Hot water heating system, gas fired. Two zones, circulating pumps. Vaillant boiler, about 20 yrs old. Pressure release valve very frequently discharging (hot) water. Plumber looked at it and says he turned down pressure and if problem continues try changing out expansion tank. Problem persisted, and I have changed expansion tank out, not problem is worse. Replaced release valve several times with new, no help. Gauge show pressure going into low 30 psi's and very high temps--170+ F. Any suggestions?

170 to 190 is okay for temp. You say you have replaced pressure releif valve and expansion tank. Are you sure your expansion tank is large enough for your system?
I would replace the pressure reducing valve next. This is most lilely the source of your problem. The new pressure reducing valve should be preset at 12psi from the factory. So your psi reading should be around 12psi when boiler is cold(give or take a couple of degrees). Once boiler starts to heat up, the pressure will rise, it may even get to around twenty to twenty five, depending on your system, if it gets to thirty, you have a problem, as relief valve is set to go off at thirty psi. If system is piped and sized correctly, you really shouldn't see the pressure get over twenty. Like I said, I would replace the pressure reducing valve, and please let me know how things work out.

Thanks for your quick response. As stated, the Pressure release valve has already been replaced. The expansion tank is the same size as the original tank, which seemed to function well enough for 18 yrs. My focus is now on the cold water intake control valve. I'm trying to determine is it is faulty or has somehow been set wrong and can be reset properly...

Dump some water out of the system and see if it re-pressurizes excessively. A faulty reducing valve would cause this.

The automatic fill valve is probably leaking water into the system allowing the boiler to become over pressured. Fill the boiler to the correct pressure and shut off the fill water valve before the auto fill assembly. This way you will pervent any additional water from entering into the system and it will also let you see if that is the problem.

Now with that said you will notice that the boiler pressure will change higher as the boiler water temperature goes up. This is normal is most circumstances.

I've done as you suggested over the last several days, stabilizing the pressure @ 18 psi and having no add'l pressure release, so I'm fairly confident that the problem is the intake valve. I have given it about 1/2 turn clock-wise, screwing the brass valve slightly deeper into the valve body. Is this the correct direction? It's a Watts valve...

Turning the adjuster in (clockwise) will increase system pressure.

Like KC13 said turning clockwise will raise pressure. If the valve is leaking it will not make a difference which way you turn it since it will keep leaking.