it seems like you have a cold solder joint somewhere that is causing intermittent failure. When pressing down on the pcmcia area you are probably causing the joint to open and of course when the cpu/board warms up the expansion is causing the intermittent breaks too.
Cold solder joints are very hard to trace. One thing to try is to use a pencil with an eraser and gently press down on the motherboard in different areas to see if you can home in on the area. To fix a cold solder joint you would have to use a bit of flux on all pads and touch up with an iron to remelt/reflow the solder. Don't just attempt to use an iron with no flux as you could dry up more joints.