From what you said, did you add the grains to the water? If so, I was under the impression its best to add the water (little at first then slowly more and more) to the grains. Something about shocking the enzymes.
I always found my temperatures to settle out after 2-3 minutes. Before I would check after about 30 seconds and be freaked out when it wasnt close and I would add either boiling or ice water then found out it was over done. So I once just waited 2 minutes to see and sure enough, I would hit the temps.
How much grain and water did you use? 160 seems a little low and I am shocked you didnt get any temp loss when you added it to the grain. There should have been some.