[ ... ] That serial port is a plug (male connector), so not so easy to put resistors on it. Plug the null modem cable in it, so you will get a socket (female connector) on the other. Grab the socket from the other end, and stick a plain metallic wire in pin 5 (GND) and 2 (RXD). Then check your boot sequence. No need for solder, adapters, rocket science schematics, etc. I doubt very much the noise is the problem, but it will not hurt to check.
One can check the control pins of serial in code, so you need to be sure the bootcode doesn't check for some assert/deassert control lines. I remember about a serial connection between 2 OpenBSD machines that refused to work with RX, TX and GND only, I had to do a _proper_ null modem cable.
