temp gauge chevy s-10

i have an 93 s-10 with 4.3 the temp gauge will go up to almost 230 if i put in nuetral it'll come back down to 180 and will be ok for few miles then go back up to 230, put in nuetral goes back down to 180 and will be fine after that,
also have 91 s-10 with 2.8 and it does same thing, i've had radiator redone new t-stat hoses radiator cap. i'm puzzled any ideas????

chage the thermistat(s) and check the sending unit