I'm thinking you have poor airflow to the rad if the capacity is sufficient. At idle and low speed, you have no issue so your fans/rad are doing their job. Def check the airflow to the rad and do your best to improve this.
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97 T/A Ram Air Convt
Forever dyno queen / 777rwhp 662 rwtq @ 17lbs / 10.2 @ 140
'24 Corvette Z06
17 Sierra 2500HD Dmax
81 Turbo TA
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