Open the top half of the amiga then put any fan above the indivision aga and try again, I had serious heat issues when I installed the mk2 and to keep it cool enough and still as silent as possible I had to use one silent 40mm fan directly above Lisa and 4x 40mm fans at the grils in the top lid.
If I turn the fans off within 5 mins I get artefacts and if I start a whdload game it kicks me to wb within a minute with a obscure error message or guru.
So im one of the unkucky with heavy heat issues so try putting a fan there and keep the lid open and see if it helps.