Let's mod a dreamer

hello, that's my first topic here on this forum, so hello all and I hope to help in future.
I ask help for a problem a with my flashforge as title I decided to convert my ff dreamer from the standard fused board to ramps 1.4 I started the project a few days ago and (surprisingly was pretty easy)I replaced just the thermal sensor for the extruders and the bed(the default are not supported on marlin).
Today I started a few tests and everything works fine, except the z-axis, when I start a print it's all normal until about the 15/16 layer then the z-axis lift down of approximately 5mm and starts to print in air.
I think that should be a firmware problem but I can't find the problem...