I had the same problem with my toyota corolla-X, everything physically looked fine, the engine, radiator,no smoke,no oil reduction,no change in oil color, but eventually the problem was that the valves got jammed and the car was performing exactly the way your car is doing, I got that problem solved with engine head over hauling from Toyota dealership at a cost of 12500/= and the car's performance is supreme. I shared my experience with you, its not that you also must have the same problem, its a possibility, the above physical investigations that you listed do not assure that the car hasn't valves problem.