I have a male Lab who has regular training and most of the time is very willing and obedient.  However, just lately - right in the middle of a training exercise, he has taken to running off.  He was bought as a 'pet' for the family and we all, including our children, keep up the basic training....  he will do established sit/stays, he doesn't pull on the leas - heels well and we are doing basic obedience work but don't know how to stop this occasional lapse in concentration.  Do I go back to basics i.e. back on the lead?    He is four years old and this problem was there when he was very young and we were training the basics - then for some reason it has crept in again.

It sounds as if he's getting bored ;)  or possibly even suffering from overtraining if the whole family is training throughout the day

How long are the training sessions?  Anything longer than 5 minutes is a bit pointless as the dog needs to actually be paying attention ;)

Having said that, you could do 5 or 6, 2 or 3 minute sessions a day - well spaced out and he'd then have time to take it in without being bombarded.  You'll find he'll probably learn faster :) 

Also try teaching him something new, perhaps he's bored of the same old, same old ;)