There could be several things going on here. First I would suggest going back through all the mechanical bits, everything should move smoothly by hand. The next thing to look at is how you have the motors tuned. Are your acceleration and max velocity values Reasonable. The Z-axis is typically set to lower accel and velocity because your fighting gravity with a heavy load.
Your motors have a relatively high inductance for the power supply voltage you are using. This means you won't be able to accelerate as quickly. It's human nature to want to use the biggest rated motor that fits your driver but this seldom leads to good performance. The motor needs to be matched to the load, the driver matched to the motors and then the right power supply chosen.
In this case you have the motors already so I would suggest a 48V power supply if you want to upgrade it. As to current capacity of the supply, the G540 has an internal fast blow 7A fuse. You will never draw more than 7A. What does all of this mean? If you are using a linear, unregulated supply it is common to 'upsize' them about 30%. This help to keep the output voltage in a reasonable range given the range of output current expected. A good quality switch type supply will be able to output its full current/voltage rating with no issues. Depending on how the switcher is constructed you might need a bit of capacitance added to the output. On my G540 system I use 320W 48V MeanWell power supplies.