Water temperature ?
 

i had a cheap thermostat in my 94 cummins and the temp went up n down so i put a cummins thermostat in it now it goes to 150 degrees and stays there is this right?

That up and down was normal. Think it had to do with where it was located? Mine did he same. A steady 150 is pretty cool. Should be at least a 185-190 thermostat?

My truck runs on the cooler side also some just do

The up and down was deff normal all my cummins truck do that, and the 150 id say as long as you have good heat in the cab, leaves more room for heat if you were going to pull anything heavy..