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Have you tried changing the thermostat.

New, (with rebuild). I used to run a 180, went back to a 190. The dog portion seems to come after 20 miles or so after warm-up. Appears to lose power the longer I drive. She runs pretty good to that point, and always idles great too, hot or cold. Just takes more throttle to maintain highway cruising speed the farther I go. God forbid there is a small incline, gotta take it down to third gear and burry the throttle to maintain.

85 4Runner - With NEW Marlin 4.7 Gears!! It's a whole new beast!