Car stereo volume decreasing and increasing over time

Ever since i bought my car(1999 Ford Focus mk1), the volume of the standard car stereo(6000cd rds e-o-n) has ben turning itself down and up again over time.

When the stereo has been off for some time, having the volume on 5 will be sufficient. After a while, volume 5 will be too low and i turn it up to 10 to get the same volume as before.
It also has sudden blips where the volume goes back to normal.

it doesn't matter if the engine is on or off

Some cars have an automatic volume control that is associated with speed or road noise. Look in your owners manual to see how your radio operates and it's options. My Corolla (2010) has that and I turned it off. My 1998 Corolla does not have that option. There's also a chance your stereo is faulty. You could have loose wires to the speakers

These sets have known issues with volume going up and down and sometimes losing all volume
everything Visteon makes is of low quality and fails, they have problems with instrument cluster(dashboard) failures too

volume switches go faulty an so do the output transistors, which are the stereos internal amps