It seems that the MIPI readout is too fast, and the CX3 internal FIFO is not big enough to contain all the de-serialized bytes from MIPI. The frame rate definitely matters but the MIPI readout speed is probably more important. I would recommend looking into 1) reducing the frame rate to one per second, 2) reducing the MIPI transmitter pixel clock rate and ADC conversion rate. These usually can be done easily by manipulating certain register bits on your image sensor. However, this would possibly incur a decrease on image quality, since the pixel storage will accumulate more noise until it is read out.
CX3 can give a maximum throughput of 2.4Gbps. Now 4K, RAW 10 can be streamed from CX3 at 20fps. Refer CX3 TRM for more details. But I would advise you to start with transmitting 10/15fps and gradually move to 20fps. You can use the CX3 configuration tool available with FX3 SDK to generate the required code.