//******************************************************************
//* RAM Definitions
//******************************************************************
idata unsigned char flags,last_line,sending,TTL,Connected;
idata unsigned char byte_read,data_H,data_L,byte_read2;
idata unsigned char high_nibble, low_nibble, high_char, low_char,resend;
idata unsigned int i,txlen,rxlen,chksum16,hdrlen,tcplen,tcpdatalen_in,dhcpoptlen;
idata unsigned int tcpdatalen_out,ISN,portaddr,ip_packet_len;
idata unsigned long hdr_chksum,my_seqnum,client_seqnum,incoming_ack,expected_ack;

......


txlen = 256 ;//ip_packet_len + 14;
//if(txlen < 64)
// txlen = 64;
data_L = make8(txlen,0);
data_H = make8(txlen,1);
write_rtl(CR,0x22);
write_rtl(TPSR,txstart);
write_rtl(RSAR0,0x00);
write_rtl(RSAR1,0x40);
write_rtl(ISR,0xFF);
write_rtl(RBCR0,data_L);
write_rtl(RBCR1,data_H);
write_rtl(CR,0x12);

for(i=0;i<txlen;++i)
write_rtl(RDMAPORT,packet[enetpacketDest0+i]);

byte_read = 0;
while(!(byte_read & RDC))
read_rtl(ISR);

write_rtl(TBCR0,data_L);
write_rtl(TBCR1,data_H);
write_rtl(CR,0x24);

byte_read = 0;
while(!(byte_read & PTX))
read_rtl(ISR);



this works. But If I go higher on txlen I have problems.
excuse the messy code...