What it sounds like is an ovalized ball in the lower headset cup. Brinnelling occurs all the time in the lower race in the back of the race nearest the frame, it seems that's where the stress loads up under impact. I see this more often on road and commuter bikes more often than mountain bikes. The shocks help the ATB's and road bikes aren't steered as radically as mountain bikes generally steering in the same spot, therefore pounding or binnelling the race and or crushing the ball bearings.
Since the headset gets tight then loosens up that is most often an oval bearing, it moves, another takes its place until it's the oval bearing's turn again. After that most common is a bad headset, then ovalized head tube, then bad fork.
I would replace the headset and that would check everything but the fork (if your head tube is oval I wouldn't instal the headset, return it and go bike shopping). While the fork is out of the bike, it is easy to guage it to see if it is straight, your bike shop will have the guage.