Bjt astable multivibrator questions

Have you considered bipolar electrolytics? Normal electrolytics only have one plate doped (if that's the right word), so they can only withstand a unipolar signal (in reverse they act as a short, if I recall correctly, which is what causes them to bulge and eventually vent/explode.) Bipolar electrolytics usually internally have two capacitors connected in inverse series, or, in some cases, both plates equally doped. You can create the same effect in a pinch by wiring two electrolytics in inverse series, like this.

  |  / +  + \  |
--| |--------| |--
  |  \      /  |

You can also get axial ceramics in values up to around 1µF, and bigger in surface mount sizes (I use 22µF 6.3V 0805 capacitors.) For example here is a 1µF cap rated for 50V.

One pretty simple reason is that the tolerance on Y5V tends to be much flakier numerically than X5R or X7R. Typically -20% + 80%

A less obvious and possibly more profound reason is that the change in capacitance versus temperature is very poor on Y5V. Typically at low temperatures and high temperatures the capacitance might halve in value.

DC voltage can change the capacitance of most capacitors a bit but Y5V are really prone to it. Typically at full rated voltage the capacitance might be a fraction of what it is when low voltages are present.

Just open this data sheet and look at the first few graphs. See also this: -