Not in the shutdown days of Law, Milloy, and Samuel, it wasn't.
Even then, yeah, it was a problem. We just managed to hold (or came back), now we are losing those games.
2004 Playoffs, the Pats gain a 24-3 lead at halftime, but the game turns into a shootout because the defense gave up 24 second half points - they couldn't stop them at all in the 2nd half.
2004 Regular Season, we almost blew a 28-17 4th Quarter lead when Miami scored back to back TDs to make it a 1 point game. We almost blew a 14 point lead against Seattle late in the 3rd quarter. We almost blew a 3 TD lead vs Cin (consecutive scores made it a 7 point game, our offense was able to keep the ball for the last 4 minutes of the game to win it.
2003 Regular season we blew a 31-10 3rd Quarter lead in Indy (very similar to last game). 21 unanswered points by Indy tied the game at 31.
2003 Superbowl, that Pats blew 2 leads in the 4th Quarter (11 points, then 7 points).
2002 Regular season we blew a 14 point 4th quarter lead vs KC that sent the game to OT, we had a tie game vs the Jets that we blew in the 4th (10 unanswered points by the Jets)
2001 Superbowl we almost lost to the Rams, blowing a 14 point 4th Quarter lead. We blew a 10 point 3rd Quarter lead vs the Broncos and lost.
Here is a break down of points surrendered 1st half vs 2nd half and 4th Quarter %:
2001: 108 PA 1st half, 164 PA 2nd half, 73 PA 4th Quarter (27%)
2002: 178 PA 1st half, 168 PA 2nd half, 80 PA 4th Quarter (23%)
2003: 115 PA 1st half, 123 PA 2nd half, 72 PA 4th Quarter (30%)
2004: 149 PA 1st half, 111 PA 2nd half, 66 PA 4th Quarter (25%)
2005: 162 PA 1st half, 176 PA 2nd half, 110 PA 4th Quarter (33%)
2006: 106 PA 1st half, 131 PA 2nd half, 51 PA 4th Quarter (22%)
2007: 137 PA 1st half, 137 PA 2nd half, 79 PA 4th Quarter (29%)
2008: 157 PA 1st half, 152 PA 2nd half, 91 PA 4th Quarter (29%)
2009: 75 PA 1st half, 75 PA 2nd half, 55 PA 4th Quarter (37%)
The only time our defense has been great in the 2nd half was 2004, however, the lack of success our defense has had since 07 is a bit scary. A lot of 2007 can be chalked up to the big leads we got, but last year was horrendous and so far this year has been even worse.
There is still plenty of time to correct our 2nd half defense, but I don't like the trend this year. This team blew plenty of leads (and some big) during the Superbowl years, so we still can pull it together or just get some luck.