'88 5000CD TQW engine knocking

[Andrew de Courcy-Ireland <andrew_ireland@scotia-mcleod.com>]

Does anyone have the answer to this?

Intermittently, the digital boost guage will stop functioning and read
0.0, or 0.1
Once this happens, the engine will knock under hard acceleration
(pinging begins at about 3800rpm).  No warning lights come on.
When the boost guage is functioning properly everything is great!
        ???