When you are on the home screen of berzerk you can hold the select button down until you see the screen flash then release the button and wait 5-10 seconds. Then it will be frenzy. Do the same to get bezerk, you can flip flop whenever you want. This also works if you have the frenzy Cart, it will also play berzerk. It's weird but it works. Not sure why it loaded up as frenzy for you but berzerk will be on your cart.
When you are on the home screen of berzerk you can hold the select button down until you see the screen flash then release the button and wait 5-10 seconds. Then it will be frenzy. Do the same to get bezerk, you can flip flop whenever you want. This also works if you have the frenzy Cart, it will also play berzerk. It's weird but it works. Not sure why it loaded up as frenzy for you but berzerk will be on your cart.
It's strange that it wouldn't work in the 2600+
I thought the 7800+ was just a reskin but sounds like they may have improved the firmware, too.
There's firmware updates for the 2600 plus. It adds more games as well as other things
Also that looks alot more like Frenzy than berzerk