An on-line beam position monitoring and regular beam stability tests are of utmost importance for the Quality Assurance (QA) of the patient treatment at any particle therapy facility. The Gantry 2 at the Paul Scherrer Institute uses a strip ionization chamber for the on-line beam position verification. The design of the strip chamber placed in the beam in front of the patient allows for a small beam penumbra in order to achieve a high-quality lateral beam delivery. The detector granularity and the low noise allow the reconstruction of the signals offered by Gantry 2 with a precision of about 0.1 mm. The frond-end electronics and the whole data processing sequence have been optimized for minimizing the dead time between the beam applications to about 2 ms: the charge collection is performed in about 1 ms, read-out takes place in 100 $\mu$s while data verification and logging are completed in less than 1 ms. The sub-millimeter precision of the lateral reconstruction allows the dose inhomogeneity of the delivered field to reach a level of 1%. This is a necessary criterion to perform a reliable patient treatment.