On my '92 Accord, the power steering seems like it's fluctuating between stronger and weaker as you turn the wheel, especially when you change the direction that you turn the wheel. Does anybody know what could be the problem? Steering rack or pump maybe?

The fluid is full, but the cap to the reservoir is missing. Could that pose a problem? Also, when I take off the power steering belt, the steering is much more linear (but obviously harder).