|Your CPU cooling fan may be speeding up and down as required. That would be an indication the unit is getting hotter. Modern computers don't do well in those confined spaces in computer desks. Try running it outside that enclosure and perform the same tasks as before to see if the results are the same.|
You may THINK there is sufficient air but all the fans discharge out the rear and any hot air is pushed back around the tower.
If the unit is NOT in a desk then is it sitting on thick carpet or on top of or next to a heat register?