Re: Marlin + RAMBo: Unpredictable moves of the Z carriage;

RAMBo only has one Z driver, it just has two connections to the same driver for when you want to drive two motors in parallel (such as with a Prusa Mendel-style printer). It seems like the Z driver may be the culprit here. An easy way to check is to swap the pin numbers in pins.h so that the Z axis in firmware points to the E1 driver, and you can then plug your Z axis motor into the E1 port. It looks like RAMBo's pin settings for Z and E1 are as follows:

#define Z_STEP_PIN 35
#define Z_DIR_PIN 47
#define Z_MIN_PIN 10
#define Z_MAX_PIN 30
#define Z_ENABLE_PIN 27
#define Z_MS1_PIN 68
#define Z_MS2_PIN 67

#define E1_STEP_PIN 33
#define E1_DIR_PIN 42
#define E1_ENABLE_PIN 25
#define E1_MS1_PIN 63
#define E1_MS2_PIN 64

So swapping the pins for Z and E1 would look like this:

#define Z_STEP_PIN 33
#define Z_DIR_PIN 42
#define Z_MIN_PIN 10
#define Z_MAX_PIN 30
#define Z_ENABLE_PIN 25
#define Z_MS1_PIN 63
#define Z_MS2_PIN 64

#define E1_STEP_PIN 35
#define E1_DIR_PIN 47
#define E1_ENABLE_PIN 27
#define E1_MS1_PIN 68
#define E1_MS2_PIN 67

If your Z axis now moves properly when connected to the E1 port, then it confirms the problem is with the Z driver. Either the chip is bad, or there is a bad solder joint somewhere, or possibly something else. If the problem is with the Z driver, and you recently purchased the electronics, I would request a replacement. Alternatively, if you never plan on running dual extruders, you may just choose to run the printer with the Z motor running on the E1 port.