temp gauge goes right down when i go faster Options

[MR-AFRO-SS2]

ok people i got this problem and all started when the rad got a hole in it so i stuck some rad weld/sealant just to get it home from work. then after i left the car for a month till i could afford a new oem rad the problem started and the engine started running rough when was cold and the temperature gauge went down when do over 30 plus engine runs rough till warms up to half way then runs perfect but as soon as i start speeding up it drops and drops real fast once get on the highway .but as soon as im stopped or going slow it clams back up and i see it at the bottom or near more than i see it at normal.
plus when cold it sounds like its coking and idles rough when cold like fluctuates be low 1k nearly stalling till settles when warms up and while its cold it judders like what happens when on and of the gas in first / kangarooing but not as bad but enough to know somethings not right but only under 3k then over it runs ok but still abit hesitant on power plus is over fueling as is drinking a lot more than it did .i like im down to 25-30 mpg

so i tried to diagnose it and replaced

the temp sensor with a non cheap one
cleaned up the cold start sensor and looking new
reset the ecu
checked all lines and no air leaks
checked for air in the rad 3 times
cleaned iscv and manifold
rad and pipes are warming up nicely like should so thermostat is working fine but will pull it and clean it up.

and even tho the car has warmed up i think its telling the ecu its still cold when the gauge goes down so dumps more fuel in thinking its cold and is hurting my pockets as is my daily

so im here asking for your help guys as im all most out of ideas

[VavAlephVav]

this thing is really small so it may warm up to temp even with the thermostat open, but if the temp goes right back down as soon as you get going down the road then the t'stat has to be stuck open.
I just had mine do the exact same thing.