Since I filled each slot in my miner I'm having also ... problems to maintain ... hashrate of each board over 30 GH ... I'm unsure if it's because of the heat ... or the heatsinks ... that are blocking airflow, ... a PSU issue? .... some how I've some boards that work on one slot better then on an other one ... does anyone has a good approach how to get ... the best postitions ... ^^
also I'm seeking good heatsinks ... for the regulator part the arctic cooling stuff is to big ... something around 5 mm ...
In my experience it's a big problem when more than 3 slots per bank are used and to have >30GH/s per board. I think it's a software issue. Any hints from the developers would be awesome. I have even tried soldering power cables form each H-board directly to the 12V connector, but no fish.
I found some sweet 35x35x6mm heatsinks. 4 of them work great per board and leave room for airflow between slots.
as for 3 slots per bank, i beleive its related to the 30A limit of a texas instruments chip mentioned by intron a while back. Its got 10-14 pins and there is 1 per bank. Most likely it simply cannot handle more then 3 overclocked cards or 4 cards at 25GH