I've heard of frequency splitting and may come back to that. I have a 10uF cap on the output, this allows me to get a bipolar signal from the circuit, perhaps that is having some kind of effect. If I do the calcs for frequency and the measured R1, R2 then it implies C is 170pF, that is a bit closer. I think I would have to take components back off to find out exactly what is going on with that.
I have just more or less completed my project, a piezo driver, it has taken a couple of months, maybe 3. I built an audio amp with high GBP first off, then had to find the right transformer, then for test build a dual power supply, I will use batteries for the final portable version, then got the 555 working. Next thing is to make up a spare in case this one decides to wilt, and finally put it all in a nice box so it looks better.