1989 Chevy S-10 cooling issue (strange)

hi all,
have some weird problem with my truck. seems it runs at a normal temperature when driving, say on the interstate. but as soon as i lift my foot off the gas to slow down, and as i'm coming to a complete stop the temperature rises uncomfortably high. say from 210 up to just under 260. (just under 210 is it's normal operating temp). then after 20-30 seconds of idling, it drops back down 50 degrees or so in a matter of seconds, resting back at normal temperature. i could speed this up by putting it in neutral and revving the engine.

water pump looks good, hoses are all sound, thermostat is good, fan and fan clutch are good and the radiator is less than 3 years old.

did a simple flush but no results from that. anyone have any ideas?

thanks

Greetings jeremy welcome to the forum.

Sure sounds like that Fan Clutch isn't keeping up. Check the Shroud, make sure that the Cluch is getting proper temp info from the radiator, or the Cluch may just be slow in responding to increase in temp.

Have a good Day !
S.W.