Glitches on ring oscillator based on discrete logic gates
This is a guess. Your circuit has unexpected feedback route via the power supply rails. You have not inserted proper decoupling capacitors, there's substantial noise in the operating voltage pins of the ICs due the high current peaks which can be expected due your slowly changing logic input signals.
BTW. for reliable results you should use components within their specified operating areas. Logic gates shouldn't be used with slow input signals. They can behave unpredictably even without super noisy operating voltage when the input is in the undefined range. I do not claim nobody has got their out-of-specs circuits working, for ex. I have succesfully used CMOS inverter as a linear amp but I've been lucky.