Hi Tom,
I don't know if this will work for you or not, but it worked for me on both my 998c SI and 798c SI with a similar problem - mine was locking up and acting sporatically on certain screens until I did this with my units:
1) I adjusted the power plug pins with a small screwdriver to make sure it's seated in the Humminbird unit with a solid connection. If they're too straight, these problems would present themselves.
2) Make sure any SD cards you have in the unit are blank or removed unless you need them. I found with mine that if I had an SD in my unit, it make it act very strangely. Once I backed them out or cleared them out, that my units ran much better.
Sorry I don't have a better answer for you. Hope this helps.