Is it burning any coolant or leaking any down the block (blown head gasket?)
thermostat not installed correctly (or cheap one that doesnt work right).
Is it burped correctly (no air bubbles)
Does the fan clutch kick on like it's supposed to?

Even with no fan clutch the air alone should cool the rad enough at highway speeds as long as there is no coolant flow blockages. If it's intermittent she might just have to drive till it heats and stop and let it cool.... if that's all she can do....

89 4Runner
3" BL, M/T locks, 33"bfg mt, bilstein, Kayline, tubebumper, toyotafiberglass panels
TBI: Elocker,3.4 w/ORS,b+b,S2Sstg2cams,arias pistons,P+P intake,TRDs/c,URDpullies+7th,downey headers,MAPECU2,WEGO WB, SupraMAF,walboro255,stg4clutch, EPaOo2 sim