I recently converted my Wanhao D6 (Monoprice IIIP) to Smoothie board as the original controller would crash intermittently.
All is fine and the machine works better than it ever has the entire time I have owned it.
However on occasion the display on the GLCD will scramble with two bands of hash across the screen if the OctoPi is connected to the USB port. If I start it with the USB cord not connected it never happens.
Thinking electrical issues I taped over the 5V+ pin in the USB connector to no avail. It is kind of a PITA to have to plug in the USB cord after powering up the Printer and OctoPi.
What kind of LCD is it? If it’s controlled via SPI, occasional scrambles are to be expected from electrical noise if the cable is longer than a few centimeters.
I have the UPS because we had one of our occasional voltage fluctuations and it took out the original controller of the printer when I was running the OEM Wanhao board.
how long between the smoothie power supply and the mains? if the mains->psu cables are too long you can have a ground loop, which would cause what you describe. I would try reducing all cable lengths even if just as a test. Also, measure voltage diff between raspi gnd and smoothie gnd, as well as between the 5v gnd of each psu and report.