98 Chevy Blazer Transmission issue

Hi. I have a 98 Blazer. Lately, when I drive with a constent speed and a seemingly flat surface or on a incline, and when my speed is between 45 - 60 m/h and the rpm's are between 1400-1700 or so, it seems that the transmission tries to get into a lower gear. The RPMs increase by about 200 and then it stays there for a second and drops back. Then couple of seconds later it tries again. At times I can hear some thumping sound coming from my transmission when that happens. There are no problems with the shifting otherwise. Only when I keep the truck in a constant speed between 45-60 m/h (incline or flat) and within the above mentions rpms. Any help is greatly appreciated.