Radio behaving strangly

I have put an aftermarket stereo into my XJ, and it is acting wierd. When I turn the volume up one click, it jumps many clicks, but when I turn it down, it goes one click at a time. No matter what the volume is when I turn the vehicle off, it is very loud when I restart it. It keeps time and remembers the radio stations, but there are various other random malfunctions. I think it is the result of a faulty ground, but I am unsure how to check that. Any suggesstions?

Probably the radio, not the truck.

I have had a couple of them do that and it was always the radio. Jensen and a Pioneer.

I was thinking it may be the radio, but I wanted to make sure before I bought a new one. The radio is a few years old and had been in the attic for a little while.