Scarborough Athletic were taken apart in the second half at Prescot Cables, slumping to a 3-1 defeat.
Having looked on a par with their Merseyside rivals, Boro crumbled in the latter stages, allowing Cables to surge to a comfortable three-point haul.
Steve Kittrick and his coaching staff surprised the travelling support by giving Danny Stimpson a start in attack, with new signing James Knowles taking his place at the back.
With the new central defensive partnership of Knowles and Sam Hewitt bedding in, there were a few worrying moments in the early duels.
Winger Harry Cain put things on a plate for Jordan Southworth, delivering a superb ball into the box, but the Prescot man fluffed his lines and lifted the ball into the hands of keeper Tommy Taylor.
Stimpson's first threatening moment saw him bring a long ball down beautifully, but he lashed his effort across goal and out for a throw-in.
His addition to the strike-force was making route one a feasible option though, and Boro almost took advantage of this when Stimpson flicked the ball into the path of Coulson, but keeper Ben Barnes raced from his line to smother.
More scares followed for Boro, with Prescot showing glimpses of why they are sitting alongside the division's cream in the top spots of the table.
Dave Merris had to head a Southworth drive off his own line and then Lloyd Dean had a looping header well saved by a stretching Tommy Taylor.
Boro had a great chance of their own right on the stroke of half-time. Debut-boy Knowles rose at the far post and angled a header at goal, but Barnes managed to make a finger-tip save to keep things level at the break.
Boro shuffled the pack a touch at the break, Stimpson being withdrawn after doing his bit up top, Billy Logan taking his spot.
After a quiet opening 45 minutes, the game took off with just a few minutes of the second period played.
Prescot took the lead, when after a Ross Daly block on the line and a shot shuddering the post, Baba Conteh popped up to roll into an empty net.
Then, within seconds of the re-start, Boro were on level terms through that man Coulson.
But it was all about the creative play of Daly, who danced into the box and put in on a plate for Boro's top scorer from a yard out. Obviously Coulson made no mistake.
Things calmed down a touch after this, though Jamie Price had an opportunity to make it 2-1 to Boro, charging on a Logan flick but driving straight at Barnes.
With the game seeming set for a stalemate, Prescot were gifted the lead with just over 15 minutes to play.
Sourthworth delivered a cross into the box, Knowles left it, which wrong-footed Taylor and the ball settled in the bottom corner.
Boro continued their capitulation a few minutes later, failing to deal with a bouncing ball in the area and allowing Josh Klein-Davies to smash home from close range.
Prescot tried to rub more salt into the Boro wounds in the moments that followed, with their pace giving Boro a defensive nightmare.
Cain was unlucky not to make it 4-1 when he bent the ball from the edge of the box, but his shot struck the angle of post and bar.
Boro finally went for it once the clock had ticked into injury-time, with Jimmy Beadle forcing a superb save from Barnes, but Cables stood firm to collect a deserved victory.