strange temp readings

I recently replaced the engine in my 95 tacoma (2rz) and now I am getting strange things happening on my temp gauge. Firstly from time to time the needle fluctuates I will be driving along at a steady speed and all of a sudden it will go up to about two thirds the way to the red line and then go back down again, sometimes when I decelerate the needle will go almost all the way to the red line and as soon as I accelerate again it will go back down to around about half way or a little over and when I am accelerating up a hill or under fairly strong power it will steadily rise and then fall again quickly when I stop accelerating. any one have any ideas? I have already switched out the thermostat and the water temp??? sensor Im wondering if it could be a timing issue? and if so would it more likely be too far advanced or too retarded? thanks in advance for any help...