I had recently replaced both fuel filters and now notice an improvement of top end performance. However, when climbing a medium incline, say 6-8% it will very slowly accelerate from 60 to 65 taking about 10 seconds at full throttle in top gear. Also this is usually the top speed for the car on such slopes. When climbing it used to surge before the new filters and actually decrease in speed, but not anymore. There is plenty of power on the flats though, it seems like it could reach a 110, but I've never tried to for fear of stressing the engine. It does pull strong past 90. Is this a typical performance profile of an 85 SD? Or does my car seem to be underpowered for some reason?