Evening scintillation is an aspect of space weather occurring primarily in the equatorial region due to scattering of satellite signals by ionospheric F-region irregularities. In order to quantify scintillation-related performance degradation in GPS (Global Positioning System), we operated single frequency (L1) Ionospheric Scintillation Monitors (ISM), sampling at 50 Hz, at 6 sites in the South East Asia/Oceania region during the most recent solar maximum period 1998--2002. Scintillation indices, horizontal position, receiver lock time and dilution of precision were recorded every minute. Data reveal that individual one-minute positional errors can increase from a few meters to 30 m during scintillation, whilst statistics at the hourly upper decile level reveal an underlying seasonal modulation from about 5 m to 15 m. Error maxima occur during the equinoxes and are more pronounced at the equatorial anomaly crests than at the magnetic equator, as expected. Such errors may concern users of systems that are dependent on GPS.