96 blazer rpm issue

Here is the problem, when driving at a constant speed and the rpm's are around 2000 the blazer will begin to jerk and the rpm's will drop to about 1500 then back to 2000 several times until gas is given, then it will calm but eventually it will occur again. This manily happens on the freeway or city streets where the speed is at least 35mph. It seems as though the truck is either loosing power or not getting fuel but I am not sure. I have done all the major tune up things thinking it could be a missfire but this did not solve the problem. I was going to check the fuel pressue next but was wondering if anyone else has ran it to this sort of problem.

Thanks for your help.

Here is the problem, when driving at a constant speed and the rpm's are around 2000 the blazer will begin to jerk and the rpm's will drop to about 1500 then back to 2000 several times until gas is given, then it will calm but eventually it will occur again. This manily happens on the freeway or city streets where the speed is at least 35mph. It seems as though the truck is either loosing power or not getting fuel but I am not sure. I have done all the major tune up things thinking it could be a missfire but this did not solve the problem. I was going to check the fuel pressue next but was wondering if anyone else has ran it to this sort of problem.

Thanks for your help.
If an automatic, I would change the tranny fluid and filter.