Random temp increase and decrease would be by-pass system, this can include heater core and its hoses and control valve.
This would be truly random, high speed or stopped wouldn't matter, temp needle would just go up and down randomly.
Yours isn't random, temp increase happens when air flow thru rad goes down, i.e. you slow down.
Heating up more when engine RPMs are high, engine under load, is a circulation problem, water pump or clogged tubes in rad, could be t-stat but they are suppose to fail open, so low temp is usually sign of thermostat issue.
Heat rising when slowing or at idle is usually air flow issue.
Check fan clutch again, the clutch on this unit is activated by radiator heat not engine heat, front of the unit heats up with radiator and engages fan more as it heats up.
Start cold engine let it run for 1 minute and shut it off, this will cause clutch to dis-engage because rad is cold.
Spin fan with your hand, should spin easily
Drive until engine is fully heated up then shut off engine and check fan again.
Should not spin, it will move but not easily.
Fan shroud is very important for cooling radiator with a fan, without it less air can be pulled thru rad when stopped.
Check front of rad make sure it is clean, good air flow.
If you have A/C then make sure condenser(in front of rad) is clean, and if it has an electric fan make sure it is working, start engine and turn on A/C, compressor should come on and in a moment or two fan should come on to cool the compressed refrigerant.