Please try [8bit4ever SX-1] + [GR8NET]

Is the power supply strong enough? Both have FPGAs that need to initialise on a cold boot and maybe they draw more power during that time.

Sound like reset signal problem. Is there circuit diagram availble for the SX-1 device? I wonder what is going on the reset signal when SX-1's FPGA is configuring and just after it - is there a pull up or down resistor on the line? What is the active low duration? Unfortubately can properly only be seen with the scope...

I do not remember I've seen specification about reset signal duration in the MSX standard, however it must be present in the documentation (at least for completeness).

Update: I did small research on the SX-1. There's no tech information available apart from pics on the internet (one from 8bits4ever website and another from MRC thread):

First, I see difference in the silkscreen, thus there must be several versions of the device. Then, looking at the board pic, I see it uses same reset supervisor M51953 as OCM. The one major difference I see it uses buck converter LM2596. Its fequency is 150 kHz which is relatively low. It features non-ceramic caps (aluminium or tantalum) of relatively high value requiring low ESR. Low ESR tantalum are relatively expensive, thus I see aluminium are used. Not clear if layout follows layout guidelines (see the datasheet), IMHO inductor is too far away from the device. Seems copper tracks on the board are slightly narrow and of wrong shape (to say the least). They most probably withstand current, but narrower tracks are subject to the noise and cross-talk.

For further reading: https://forum.arduino.cc/index.php?topic=133138.0, https://electronics.stackexchange.com/questions/251914/lm259... and http://www.bajdi.com/testing-switch-mode-voltage-regulators/, it is very important where the components - in particular capacitors, were sourced.

No information about power supply requirements on the main page. Which power supply is used? Is it provided with the board?

I need more technical info on the device to make complete analysis.

SX-1 and GR8bit works well for me. Got the green board one and a (very) cheap power supply.

Of course I meant GR8NET, GR8bit s waiting for its completion, hope to get it working this winter...

We've tested most of the cartridges we produce (Carnivore2, SD-512, CF-640, FDC-600, FM-PAK, MSX Blaster) with the SX-1 board. We did not find any problems so far. Also tested some original ROM game carts, but nothing special with those. All test were made using a 9v DC (outputs 12v unconnected) center negative PSU. The unit can take up to 16v DC, but is best to keep the DC input between 9-12 volts.