Car gurus...

[MarkS]

I have a '98 Nissan Sentra.

In the past three weeks, I've changed the rear brakes, front brakes, crankshaft oil seal, spark plugs and wires, air filter, oil and oil filter, and the right side drive shaft (split boot). The problem I'm having is that when I'm driving on flat, smooth road, the steering wheel oscillates from side to side. If I take my hands off of the wheel, the car remains straight, but it looks like a child is pretending to drive. It has been doing this for months.

I thought that this was due to a ball on a tire, but I've had all four tires off and there is no ball. I thought it was the drive shaft, since a split boot will also cause this, but replacing it made no difference. While changing the front brakes, I noticed that the boot on the left side drive shaft is also cracked, but this is a rather fresh crack (the boot on the other side was split in two), so I doubt that it is the source of the problem. The front and rear struts are original to the car (and in bad shape) and were next on my to do list, until I saw the left side drive shaft. I don't think the struts are the issue here as this problem shows itself on smooth roadway.

What could be causing this? I can pretty much fix or replace anything on the car, short of the block, but I would have to know what to fix first. Any ideas?