Assistance is requested as I am out of ideas. My '88 240 wagon when operating at 70-75 mpg and climbing a long grade; on pressing down the accelerator to maintain speed or accelerate; after a short period the water temp will climb to the 2/3 to 3/4 range. At the same time the engine power will drop dramatically regardless of accelerator position down to about 50 mph. There is an accompanying pinging heard. After slowing down on level ground & cooling down, the pinging stops & power is normal. Around town there are no problems at all. The timing has been checked as I thought that to be the problem, but it was right on target. The plugs are in good shape. Distributor not properly advancing? Knock sensor bad? Partial exhaust blockage of some type? I would appreciate any suggested solutions.