One case does not imply that the circuit is sensitive to capacitor type. Try testing with ten of one type and ten of another (all same value and ideally tolerance) then report back.This one is a skull scratcher to me.
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Simulation gives 70Hz, so 5Hz is just 7%.You don't say what the intended output frequency is (???), so this is guesswork. Assuming it is around 50 Hz, 5 Hz is only 10%.
Thanks for the refresher course. I forgot about the tolerance varyible.Electronic components such as capacitors, resistors and inductors all have a tolerance associated with them, an error band surrounding the part's design value within which the actual value of any given part is guaranteed to lie. For example, the actual value of a 100 Ω, 5% resistor may lie anywhere between 95 Ω and 105 Ω. The actual value of a 1 μF, 20% capacitor may be anything between 0.80 μF and 1.20 μF.
Expecting any two identically-marked capacitors to be exactly the same value is unrealistic. So the fact that your output frequency changed when you replaced one 1 μF capacitor with another 1 μF capacitor is hardly a head-scratcher.