Saints slipped to a fifth defeat of the season on a weekend that saw Finedon Dolben strengthen their grip on the lead at the top of the Hevey Building Supplies Northamptonshire Cricket League.
Rushton became the fifth side to beat Saints this summer, but after being dismissed for 143 they did not look like they had much of a chance.
That was even more the case as Dan Harris (55) and Geoff Jackson (33) put on 94 for the first wicket in reply.
However, the Saints then slumped dramatically and lost all their wickets for just 48 runs.
They were eventually all out just one run short on 142 as Jonathan Carpenter bagged 6-61.
Finedon stretched their lead with an eighth win of the season, this time at the expense of Old Northamptonians.
In a high-scoring encounter, Ben Howgego (69), Chris Todd (49), Greig Hofbauer (87) and Jack Chopping (48no) were all in the runs as Finedon posted 288-5.
ONs made a good stab at chasing the score down but ultimately fell 21 short as Tom Brett claimed 5-86.
Defending champions Peterborough are now the closest chasers after they crushed Brixworth.
Asim Butt (115) and David Clarke (93) put on 210 for the first wicket to take control from the start.
Having declared on 267-5, Peterborough then bowled out Brixworth for 151 - despite 57 from Tom Worthington - as Paul McMahon took 4-40.
Petrus Jeftha was to the fore for Oundle once again as the Premier Division new boys picked up another success - their sixth of the season having bowled first.
Jeftha’s 7-39 helped dismiss Wellingborough Town for 122 and, with Rick Andrew hitting 56 in the reply, Oundle were winners by five wickets.
Tyler Lorton was the star man as Stony Stratford inflicted more misery on fellow strugglers Burton Latimer.
He and Steve Bellew both took four wickets in Burton’s 159 all out and then Lorton made 82 to guide Stony home.