Replacing BT 2.0 with BLE 4.0, strange behaviour

Hi,

I'm working on a Timelapse slider project which - in short - uses an Arduino pro mini 3.3v board, bluetooth module, Nema 17 stepper motor and a BigEasy Driver.

I have recently replaced a DFRobot 2.0 Bluetooth module V3, with a Bluetooth 4.0 HM-10 (to make the whole thing compatible with iPhones) and it seems that the new BT module is affecting the stepper motor or BigEasy Driver :o

Each time I send a message over serial port by Serial.print() (BT module -> phone), the stepper motor loose steps/speed for a milisecond :confused: It didn't happen with the previous DFRobot 2.0 Bluetooth module.

What might be the reason?

Kindly,
Artur

EDIT

Diagram:

I have seen a video about a similar problem with timing issues on the Arduino. Do you use delays to turn things on and off in your code? It could be that your Bluetooth module takes longer to initiate and decode your commands, thus affecting a delay in your code. The guy used some sort of internal clock in the Arduino to do all his timing commands with. I'm gonna search my Youtube history, but like I said, it was quite a while ago.

No, I don't use delays. I use SimpleTimer library for timing.

The thing is, that I didn't change anything in the code after replacing BT 2.0 with 4.0 module.

Whenever I use Serial.print(), the stepper motor drops speed/steps for a tiny bit of a second.