Support material under extrusion

For some time now I'm having this problem, my extruder is precisely calibrated, it's just right for any print I make! Except when I need support D:

Ok, I should just increase the settings a little bit, but this just ruins small delicate prints.
But as it is, the first support layer is too tine, does not stick to the plate. The other layers get so under extruded that there is no material deposited at all on some places. And in tree or four layers everything start dancing around stuck at the hotend nozzle.

Any guesses?
I'm using Marlin and Slic3r on a homemade Prusa i3.

Slow down the first layer speed or use a raft. Slic3r support structures uses too small areas that can get loose or fall over easily. I sometimes design support structures into the object, which can improve the print quality, time and reduce wasted filament.

I did tried raft on one occasion. The result was so disastrous that I never looked to that option again
My problem is really with under extrusion, I feel it would work out ok if I could just tweak the volume for support material only.

I saw some models with "built in" supports, but I have no idea how to do it. How to tell on OpenSCAD the amount of extrusion for a part of the model?
The idea is to minimize the amount of material, right?!