The steering wheel shakes when coming to a stop?

About a month ago, the steering wheel began to shim when coming from a complete stop and accelerating slowly. The shim was very pronounced in the steering wheel but not felt in the rest of the car (like a high speed front end shim). The shim appears to occur when it is still cold and within the first few miles. The speed range seems to be in the 10-20 mph range. As you accelerate, and the car is driven further, the shim dissipates. Until present, the problem is has only been sporadic. Also, no shimming is experienced at normal or highway speeds. The steering pump fluid level is normal and belt condition and tension appear good. If any additional information is needed, please let me know. Thanks in advance.