1998 Dodge Ram I need a diagnosis

Engine Performance problem
1998 Dodge Ram 4 cyl Two Wheel Drive Automatic

im not quite sure if it is the engine or not,
but I get to 20mpr [25 at most] and it will rev up to just about 3000rpm but will not go any faster. If I do try to go any faster, the engine makes a rattling noise.
the engine is overheating enough to make the water boil, but it idles just fine.
the oil and everything in it is just fine.
do you have any ideas of what could be wrong?

please and thank you.