When the heating is working the PSU may be over-stressed and this may effect the drivers/currents.

However, not all steppers and drivers are made equal.

First look up your specific stepper motor types and which current they actually need. Locate the type number on the steppers and google them. Different steppers require different max currents as the coil resistances differ. I have steppers that have a max current of 1.2A, 1.3A and even as high as 2.5A.

Once you know the max current your steppers need, then adjust the pots on the driver boards. Be aware that there are boards out there with different current adjusting resistors (100 and 200 milliOhm), best to locate them and measure to be sure.