After trying a lot of things to solve the noise issue from the DC motor, I finally decided being without ideas to replace the DC motor. And guess what. Now the infrared reader keeps receiving correct values. The big question is now of course what is the difference between those 2 DC motors. Maybe too much Volts ? As a next experiment I will put use less AA batteries to support the DC-motor. At this moment I use 6 Volts (5 AA batteries of 1.2V). I have now ordered batteryholders (4 AA, 3 AA, 2 AA batteries) and will verify whether I see any change with the DC-motor I initially used.
I will come back with more news as soon as I got the battery holders from China. Might take a while.