Hello,
This might be a digitizer problem - used to affect the first touch screen blackberrys - and the blackberry I had behaved similar to this.
Has the device suffered water damage, or been dropped? Is the screen unblemished with no cracks?
Something to try - type with the keyboard horizontal and try typing with the phone upside down - i.e using another part of the touchscreen to type. Does the problem go away or greatly reduce? If it stays exactly the same, (no improvement) then it could be a software issue - has anything changed in the last few days, new phone updates? New apps installed?
Try selecting a different keyboard in the settings if possible, see if that makes any difference.