I had an exactly similar issue in the past, it was due to the RS232 Gnd which
was not correctly connected to PC Gnd. I simply added a wire between RS232
Cable shield (from PC) and the board DB9 connector shield (or Gnd, the same)
and I found it was solving the issue. So I just re-soldered the connector
For your case, when using the probe, you connect the Gnd of the USART to the PC
or Oscillo Gnd and the voltages are stabilized.
Retrieving data ...