For the last year my 520 has started 1st turn of the key,it fires up at 1200rpm and immediately settles to 700/750 rpm hot or cold. On Saturday it started first turn of the key as usual but dropped from the 1200 to less than 500 and almost stalled but then settled at 750 as usual. This happened again on Sunday but this time it did stall and I almost flattened the battery getting it started again. Has anyone experienced this or knows what the problem is? By the way, once started it runs and starts perfectly during the day.