Taz has become the dog from hell again off lead

within the space of a couple of weeks.
We regularly go to a nearby nature park, with lots of tempting scents. Until the last couple of weeks, Taz would go off and have a sniff but would return within a couple of minutes, the last couple of weeks he has been really bad, going off an not coming back for ages then when he does come back if i look towards him he tears off again the opposite direction until he is ready to come back.
I have gone back to basics so many times, taken it steady and not moved on to the next step until he was ready, etc., but it seems he gets to the point where he is reliable, sticks to it for a month or so, tricks me into a false sense of security then reverts back to ignoring me. In the house he is a dream, and the last couple of months he has been brilliant at agility (he used to disappear there too), but this time he is worse than ever. He also has this "look" and when he stands there with this "look" on this face, i know that nothing i do will stop him from doing a runner. When he does finally come back to me, i dont shout at him, just put him back on the lead. Fortunately, my lab (10 months) does recall.
Its really starting to limit our off lead walks, as i cannot let him off at any of our usual close by parks in case he goes in the fields (where the farmers will shoot). I am really starting to think our only option is to keep him on lead and accept defeat.
I just wonder if this could be a result of a really distruptive summer we have had - we have had a kitchen extension which has meant that we have had a lot of workmen around and Taz doesnt really like strangers (although he seems fairly happy iwth them) on top of this i had a major event at work about 3 weeks ago, and prior to that had had to work long hours (sons or hubby where around most of the time so the dogs werent left unattended). He hs also seemed in a strange mood, and in a morning hasnt been keen to come for a walk. Any advise?