Potterton suprima APS problem

Hi. I have a Suprima 50 HE. When the boiler starts up the usual sequence commences and the APS operates and allows normal ignition. After 30 seconds or so the boiler cuts out and after 4 repeats it locks up. I have kept a multimeter directly on the contacts of the APS and what is occurring is that after 20-30 seconds of operation the resistance across the switch gradually increases . Any body with any ideas please ?