As the sympathetic nervous system alos deals with normal stress, it may 'kick in' and give the over training symptoms at different points if there is other things causing problems - stress at work, kids, etc etc.
Only thing is to rest......oh and Guiness, always guiness!!
Symptons can be:
Behavioural
Apathy, lethargy, poor concerntration, chnages in sleep patterns, irritability, decreased libido, clumsiness, increased thirst, sluggishness, sugar cravings
Physical
Reduced performance, weight changes, morning heart rate changes, muscle sorenes, swollen lymphs glands, diarrhe, injury, infection, amenorrhea, decreased exercise heart rate, slow healing cuts
This is just bits and bobs taken from various books - there seems to be a lot of different opinions, but the above a summary.
Hope it helps 