1998 Lexus ES 300 Steering

Intermittently the steering wheel is hard to turn. I've only had the car for about 4 weeks. The first 3 weeks, the steering was fine. Then all of a sudden, it felt stiff. Not in all positions, though. There are spots where it is stiff. The spots are different, I think. I may be wrong about that. In a turn, there are parts where it is easy to turn and parts where it is very hard. Then when you get past the hard part, it is easy again. Then the steering felt normal for a day or two and then it started feeling stiff again and would feel normal 5 minutes later for a few minutes. It seems very sporadic. I've flushed the power steering fluid and topped it up with the correct ATF Dexron III. The first time I checked the fluid it looked like engine oil: golden brown. That is why I flushed it, but no difference. A mechanic told me it might be the intermediate U joint shaft on the steering column. He said they get stiff. Wouldn't it be stiff all the time, though? Any ideas?