Hi guys 'n gals

So i recently bought my first 3d printer, a Delta printer.
First few prints off of it came out pretty good already, had some trouble with stringing but that was quickly fixed.
Now i ran into a new problem with the longer prints... after a while it seems that the layers shift, not the same every time but always in the same direction.

After googling that problem I found a few possible solutions one of which was checking the pulleys on being fixed or not.
Those still seemed to be fixed.

Second possible solution was turning down the pot-meter on the stepper drivers because they'd be getting too hot.
Instead of doing that I thought I'd try cooling them instead with a little fan, but without success.
Well... partially i think, the shift wasn't as big as it used to be but was more frequent.
Instead of one big shift it had 2 smaller ones in roughly the same amount of layers.

Now as I said, I'm a bit hesitant in medling with the pot-meters on the stepper drivers.
Could there be any other causes for my problem?

Hope someone can help me out,
Rick