IMHO with the weak stepper drivers supplied, you need as much cooling as you can get, the heat sinks on mine got red-hot before I installed a 120mm A/C fan.
I since replaced the ramps with a duet 2- they have no sinks because they are rated for high amps and are soldered to a thick board to dissipate heat- but I still install sinks.
Another idea is to check to see how tight your belts are, and, since you got an extra stepper driver, swap and adjust it with the new one.