Sounds like it wants more revs in order to stay cool. Wonder if the water pump is in OK shape? If its a crappy one the impeller can start slipping on the shaft and provide reduced coolant flow. Driving in 4th should not help it stay cooler if a blown HG is your only problem... if anything that would make it worse, by forcing more air into the cooling system.
Driving it won't hurt it as long as the temp stays in check -- but don't let it overheat, or else your head will be toast.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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