Printing parts with holes in between the infill

Good morning to everyone,
I started to build my reprap about two years ago, but my work and the difficulty to find parts in Venezuela were keeping me from build it until I finished it three months ago. Then the problems started, wobbling bed, poor built hobbed bolt, Falling Z-axis. But now, finally i'm pretty sure i'm close enough to make my first perfect part. The problem I have is that every part I print has holes in the infill. Doesn't seem to follow any pattern and doesn't look like a problem with the extruder(I've tried checking the extruder calibration with no problems at all). i have tried changing the Width over thickness without any results and basically I'm running low on ideas. I'm posting this hopping that you can help me with the problem.

The holes does not seems to appear in the perimeters just in the infill. and doesn't looks like a temperature problem because increasing the temp does not improve the printing at all.

Attached you will find pictures of the problem

Regards

The incomplete reprap beginners guide. [reprap.org]
Object Calibration, alexrj/Slic3r Github [github.com]
Machine Calibration, Triffid Hunter's Calibration Guide [reprap.org]
Commissioning, Calibrating [reprap.org]
Calibration [reprap.org]