First off: 1990 240DL 119,000

The other day I was on a 200 mile trip and after about an hour or so the engine temp guage moved up to about an eigth of an inch below the red line. I turned on the heater full blast and eventually the needle went down to about the middle again. It seemed to happan again when I was going around sixty and then accelerated to about eighty. Again, I turned on the heater to draw heat from the engine. The needle moved to the middle again and stayed there. I would say on the whole three hour trip this happaned about three or four times. I tried to determine what was causing the needle to shoot up - I thought it was when I was accelerating suddenly, but it happaned once when I decelerated quickly at a toll booth. The needle never got into the red zone - but it got up there - pretty close. What could be causing this? Bad water pump? Bad thermostat? I flushed and filled the radiator around 5,000 miles ago, I just changed the oil before the trip. I think it could be a failing water pump. Thanks for your help.