Hi Charlote,
There may be a hardware problem with the PITCH BEND (PB) wheel, which means that the mechanical center position (no change in pitch) is no longer correctly recognized.
First, please bring the MODULATION wheel to the normal position (turned fully down toward you), so that this wheel cannot affect the sound in any way.
Then use the PB wheel while playing, i.e. move it up and down many times. The problem may then go away, at least temporarily. If this is the case, the internal PB potentiometer or the entire PB assembly may need to be replaced. Or maybe it's "just" a mechanical problem with the spring that centers the PB wheel.
By the way, with the internal Test program you can check if the PB wheel is working properly. If you want to carry out this test yourself, you can contact me via PM (Personal Message) and I will send you the relevant information.
Best regards,
Chris