Grumpy_Mike:
The problem is simply that sometimes the module's output frequency doesn't change.
That code will only set the modules output frequency to one value once per run only. Therefore if you run the code again it will not change. Only when you power it down will it change.
but sometimes things work.
That points to decoupling problems. Where are the output wires going to? Maybe this is causing interference that is being picked up. I would add extra decoupling capacitors to the supply.
Mike... Another almost identical program sets it to the lowest frequency. High=0x00 and Low=0x00. Even if you send the same code, Wire.endTransmission()should not return an error.
The output wire - the CLK signal - goes to the scope only. Its about an inch long. In fact all the wires (there is no CLK~ or OE wire) from the board go to the breadboard and they are also about an inch long. For this current situation, the supply is the UNO board itself, and the module has 4.7uf and 0.01uf caps from the supply to ground, on board. What size cap must be applied in addition and where? All components are surface-mount technology.