Ashdown Mag250

I have an Ashdown Mag250 4x10 combo. I purchased an 8ohm speaker (ashdown mag410t). When I connect the speaker to my combo (with a normal jack), I adjust the volumes by controlling the LED of the combo, making sure that the pointer does not go into the area. red (so I adjust the input) and everything sounds perfect even when I insert the distortions because I use the low dist a lot. only that every now and then my combo seems to go into protection so I have to wait a bit to start playing again and I also have to disconnect the speaker: which seems very abnormal to me...I attach the diagram