Wild temp fluctuation when connected to Pi / Octoprint

Hi all,

I have a heavily modified CTC I3 Prusa Clone running an E3D Tornado Bowden set up. It prints great but I wanted to set up one of my Raspberry Pi's with an Octoprint server to print over the network.
I have got Octoprint all set up on a RPI 4 connected by USB to the GT2560 board on the printer. It seems to connect great except for 1 thing... I can't get the hot end temp to stabilise and it never starts printing. Set to 230 for my PETG printing, it fluctuates from 225 to 235 constantly.

At first I thought I should re-tune the PID which I did and it made no difference so I went back to my original tuned PID settings.

As the temp has always been completely stable before hooking up the Pi, I'm thinking the new Pi and Octoprint is causing the issue?

With the Printer plugged in to the Pi and Octoprint, I get wild temp fluctuations on the hotend. This is also shown on the LCD of the printer. As soon as I unplug the Pi from the printer, the temperature is completely stabilised at 230. If I plug it back in, the temp fluctuations come back.

With this in mind, I don't think it can be the printer hardware or hot end kit. I also have a good quality USB cable

The printer is also running marlin 1.1.8 at present

Any ideas?

With the exception of temperature reports, there isn't any real-time communication going on between the RPi and the printer. One gcode command is sent each time you set a temperature.

The Terminal tab of OctoPrint should show everything that is going back and forth over the serial connection, and OctoPrint can have the serial.log enabled so you can capture the gory details (and post it here).

I'd suspect some sort of electrical issue, a ground loop or unstable power. You can put tape over the 5V pin in the USB connection to see if that helps.

Thanks. I may try turning on the serial logging to check. It does seem like something is interfering. The Pi has a good quality PSU, as does the printer. So far, I have only narrowed it down to the fact that as soon as the usb connection between pi and printer is active, it starts fluctuating at printer level.
Will also grab some tape and cut off the 5v pin.

Update: Thanks for your help Brad. I've taped off the +5v on the USB lead. Worked perfectly and I now have a completely stable hot end temp.