Tempering Chocolate problem (pictures inside)

Hello,

I'm trying to mold slabs of dark chocolate and I'm having problems lately.

It's not the first time I do this, I know how to temper chocolate, but I have a new tempering machine (water/bain-marie) and maybe I'm making mistakes.

What I use:

Chocolate: Cacao Barry Dark Couverture Mi-Amer (58%)

Machine: dr TF20 (http://www.dr.ca/tf20-chocolate-tempering-machine.html) + Moulding wheel (http://www.dr.ca/moulding-wheel-for-tf20-chocolate-tempering-machine.html)

What I do:

1- Melt the chocolate overnight (at around 40 C)

2- Dial back the machine temperature to 32
3- Seed with room temperature (21-22 C, I know it's not 18-20 C which is ideal) callets

4- Start the wheel to stir + manually stir

5- Reach 32 C

6- Make my test

7- Test is ok

8- Start moulding in trays (I make slabs that I break in small pieces after)

Now usually, the first few trays give perfectly tempered chocolate and after that I get those:



This looks like chocolate that wasn't tempered correctly and was too hot when poured.

What I don't understand is why it's perfect on the outside and not it the middle, I get it's a little bit hotter there, but t the point of making that big of a difference?

Could it be because I use trays and they are not suited to chocolate moulding? (I did it in the past without problems, but those are pretty flimsy)

Could it be because I use a wanter temperer and when I go from 40 C to 32 C, the water temperature doesn't drop as fast as an air temeperer and it reheats the chocolate in the bain-marie while I'm working?

Any help would be greatly appreciated, I'm getting tired of remelting half of my batches, hehe.