print quality problem

I'm new on this forum, I make 3D prototypes that I create with the great OpenSCAD software and print with a Creality CR-20.
Recently I noticed a problem with the quality of the printing which I haven't found an answer on the net. It is the formation of small bubbles on walls. Then during printing the printer stops at times for a few seconds and then resumes printing. But this is not when the bubbles occur. Another curious phenomenon is that by printing the same part several times the bubbles tend to be positioned on the same places. See the example on the image of a 20 mm high object.
I also wondered if the summer temperature in the room where I'm printing (about 29°) could have an influence?

Attachment 14899

If anyone has an idea of what the problem might be, thank you very much in advance.