1995 Toyota 4 Runner problems with the engine running rough.

1995 Toyota 4 Runner mileage: 138,000. Last winter I started having problems with the engine running rough. Symptoms were usually: run fine at first then three to four miles later it starts running rough (esp between 45-50 mph) would run better if speed changed to higher or lower. Repair shop over several visits did the following: tune up with new plugs, flushed fuel injection system, Oxygen sensor replaced, EGR valve replaced, new distributor cap, new plug wires. Symptoms continued and were worse through the winter with colder weather. Note: symptoms either would disappear or were milder with warmer weather. Not a lot of missing during the summer except when cold, damp, rainy. Now we are back to winter and symptoms are back and are worse. Runs rough most of the time when I start it up. Sometimes it smoothes out within four or five miles, sometimes not. Sometimes when I start engine it runs smooth but starts running rough in about three or four miles. And then continues rough for rest of drive. Last winter seemed cold related and speed related. Now can run rough at any speed, still worse when cold but also rough if 45 degrees or higher. When the engine is running rough, acceleration is extremely bad. Note: the tachometer is stable when running rough. We are tired of replacing parts.