96 sidekick - intermittent power loss

I have a JX 1.6L that suddenly is experiencing intermittent power. Sometimes i can cruise along at 110 km/hr and then all of a sudden a can't keep it above 80. I try shifting at high RPMs into 5th but it can't maintain the speed and before you know it I am back into 4. This started happening about 3 months ago. I took it to a mechanic (the engine light is not on). He replaced the distributor shaft and when that didn't work he put it back. He had the exhaust back pressure checked - it was OK. He put new plugs and wires in it. He set the valves. He is baffled. When it is running poorly, it gets terrible fuel mileage. Any Ideas? Anyone? Thanks.