Serial data writing problem

I write to 16-bit RAM.

The message I write is

CONT_WRITE ADR_MSB ADR_LSB DATA0_MSB DATA0_LSB …. DATAn_MSB DATAn_LSB

where

CONT_WRITE –command - continuous write .

ADR_MSB ADR_LSB -start address. (sspi_cont_addr <= ADR_MSB & ADR_LSB)

DATA0_MSB DATA0_LSB – a pair bytes of data.

---------------------------CONTINIOUS WRITE-------------------------------------	
					 
when ST_CONT_WRITE_1 =>  --first address and data
    SITAL_1553_1_ADR <= sspi_cont_addr; 
	SITAL_1553_1_CS  <= '1';  
	SITAL_1553_1_WR  <= '1';  
	SITAL_1553_1_RD  <= '0';
					   
	st_count := 0;
	byte_idx := 0;
	
    --skip command and address
	if (bytes_counter = X"03") then
	    SitalState <= ST_CONT_WRITE_2;
	end if; 		
				
when ST_CONT_WRITE_2 =>	
    if (cs2 = '0') then 
        if (new_byte = '1') then
            --next data
            if (byte_idx = 2) then 
                byte_idx := 0;
                --next address
                sspi_cont_addr <= sspi_cont_addr + '1';
            end if;
									 
            SitalState <= ST_CONT_WRITE_3; 					
        end if;						
    else --end of message
	    SitalState <= ST_SITAL_IDLE; 
    end if;
						  
when ST_CONT_WRITE_3 =>	
    --set a word from bytes
    case byte_idx is
        when 0 =>
            sspi_cont_data(15 downto 8) <= sspi_data;
            SitalState <= ST_CONT_WRITE_2;	 
        when 1 => 
            sspi_cont_data(7 downto 0) <= sspi_data;
            reg_count := 0;
            --a word is complete						 
            SITAL_1553_1_ADR <= sspi_cont_addr; 
            SITAL_1553_1_DIN <= sspi_cont_data;
									 
            SitalState <= ST_CONT_WRITE_4; 
        when others => 
    end case;	 
    byte_idx := byte_idx + 1;
						 				  
when ST_CONT_WRITE_4 =>
    reg_count := reg_count + 1;
	
    --set the address and the data
	SITAL_1553_1_ADR <= sspi_cont_addr;
	SITAL_1553_1_DIN <= sspi_cont_data;
	
    --expose the data on the bus for x ckocks
	if (reg_count = 8) then 
	    SitalState <= ST_CONT_WRITE_2;  
	end if; 	  

I see all words in RAM. But sometimes some word is ''broken''. Say instead of 0x8000 I see 0x8100. I send 30 byte (15 words). Usually I see ''broken'' words 2 and 14, sometimes others.

What possibly can be the problem?