NHD-0216CW-AB3 unstable SPI operation on 3,3V supply

Hello everybody!
I am developing a product, incorporating the mentioned display. On the very first prototype I used a STM8A-Discovery board (STM8AF5288) connecting the display via SPI and supplying it with 5V. This worked just fine up to 250kHz SPI speed.

The first productin prototype is fitted with a STM8AF6266. The difference is the supply voltage, it is supplied with 3,3V. Which the NHD-0216CW-AB3 should be capable of (having the REGVdd tied to GND and writing 0x00 to 0x71). Other than that everything is the same. 

With that configuration, I got the display showing the desired content, but it keeps flickering, ROM table shifting, and somtimes only showing the first line.

I tried lowering the rate down to 50khz, checking the signal integrity with Osciloscope and Logic analyzer. SPI wires are short, placed serial resistors against ringing, left them away, coupling capacitors 100n, 3,3u and 10u on the supply voltage. But no luck the display besides flickering even somtimes freezes (shows static content) and comes back to life.
One thing I noticed, that when it displays only one line, the brightness is higher.

Does anybody know what else to try? Or what might be the cause?

Thank's everybody!