Pompey passed up the chance to narrow the gap to the League Two play-offs with a 2-2 draw against Cheltenham Town.
Matt Tubbs and Ryan Taylor again struck for the Blues to continue their fine goalscoring form.
But it was a disappointing night as poor defending let Andy Awford’s side down in a breathless first half.
The news a win would have narrowed the gap to the top seven to six points was a kick in the teeth for the home fans in a crowd of 13,558.
Eliot Richards and Will Packwood got the goals for the strugglers from weak play at the back from Pompey.
The first half proved a frenetic affair with four goals, clinical finishing and kamikaze defending.
The opener was all about Andy Barcham’s determination and pace as he got to the byeline and put the ball across for Tubbs to tap in.
But horrendous defending let the visitors gain the lead with two goals in five minutes.
A mix-up at the back allowed the ball to be worked Shaun Harrad, who fed Richards to finish.
It got worse after 23 minutes when a routine free-kick was lumped into the box and Packwood was left all alone to finish.
The home crowd were getting irate at the way things were going but that changed two minutes before the break.
It was Butler who delivered the free-kick for Taylor to rise high and nod home.
After the pace of the first half, there was little in the way of action after the break.
A Jed Wallace free-kick was fired at Trevor Carson after Tubbs was fouled.
And then Richards had a speculative effort gathered by Paul Jones.
The home crowd were lifted by the introduction of Wes Fogden with 22 minutes left.
But he couldn’t do anything to inspire Pompey as the game played out without major incident on a frustrating night.