My previa has been increasingly losing power for about a month. I live in the mountains, the last time I came up, I couldn't get over 20mph on the steep grades, and it was overheating, so I stopped many times. When I did get to 30mph in 1st gear an extremely loud noise came from the head area, like a piston running into the valves. At 4000 feet elevation, my compression is 140, 120, 140, 140. I removed the valve cover and everything looks ok. I pushed on each of the valve springs and they appeared to have the same pressure. Previously I had 3 codes, (302,402,420) I replaced the EGR temp sensor, the codes were cleared, I have only driven it about 20 minutes since. I had thought I might have a collapsed converter causing my power loss, but I can't relate it to the noise. Please help. It has 200k miles on it.

Did you check the timing?
Air filter?
Spark plugs? (Are you running rich or lean?)
Oil and coolant level?