tr hotter but not boiling hot gs from 150 to 240 back and forth sets around 210 most time

I have a 98 dodge ram I bought week ago used it has the v6 3.9 ltr engine in it it begain to run little hotter then normal after a friend had look in radiator and seen no water so filled it up the jug on side was completely full next day drove it it ran up to about 215 220 then goes down to normal temp 190 195 then it goes back up it will go as high as 240 or so then goes back down some times to 210 other times lower put new thermostat in it  thinking it was bad gasket was trash and so was therm so thought I found prob but it did same thing next time I drove it  then my son drove it he said it never went past the 195 mark what do you think is the prob o and when I check to see if motor is hot its not its normal temp even when it says 240 I can hold the top house well its not burning me can you tell me what it might be going on with it