Well first off, if it FRAGs when the chip was disabled, it will FRAG with the chip out of the box, you broke a trace somewhere while you were soldering (most likely your D0 point). If you were able to get the box to boot from the chip then you should reinstall it to get it to work right. You were right the first time, it isnt your soldering of the LPC points that is causing it, it is most likely your D0
When an Xbox boots up normally, it doesnt ever check the LPC points. the way the modchip works is, it GROUNDS out the D0 point, which then FORCES the xbox to bypass its orginal bios (TSOP) and boot from the LPC points (modchip)
SO let say you broke the trace AFTER the D0 grounding process, then that means your box will not start up normally without the chip. BUT since you have the chip in and disabled, the xbox will boot off the chip because D0 is already grounded (when chip is enabled, the D0 is grounded).
So what to do now? first Fix your broken trace, find it and fix it, if you dont know how, then i suggest you send it to a professional. or if you dont care about Xbox live and want to always boot from your chip, reinstall your chip and get it work. once it does READ THE FORUMS AND TUTORIALS on how to set up EVOX.
there are PLENTY of tutorials for you in the tutorials section so go READ THEM, especially the n00b guides.
good luck