image distortion generating VGA signal on STM32 f4 using DMA
OK, answering my own question because I finally got it : the FIFO feature of the DMA was not enabled, so any interruption or memory transfer was delaying access to the memory bus by the DMA.

Enabling it is much better now. This has been done with this : 

// Enable FIFO (see p190 of ref manual)
DMA2_Stream1-&gt;FCR=DMA_SxFCR_DMDIS; // disable direct mode