Just in case, if it is any "hang ups" with the microprocessor (doubtful), get the "keys" at Walmart and pull the unit out of the dash.

Now, unplug the connector and leave it out for at least 1/2 minute. This way, you KNOW that you have "unpowered it".

But I suspect it's dirty/worn button contacts and a bad volume control (also dirty/worn).

Personally, for various reasons, I like the OEM head unit. There are several members who have lightly-used ones for sale (as mentioned above). BMorgan had one at one time, others do too!