2000 F150 5.4 when at temperature hesitates and stumbles radiator stayed under pressure for 2+ days even when truck off and sitting outside in 30 degree weather. Thermostat changed and all hoses check ok changed antifreeze and still stumbles. May be two problems but seams to be temperature related. Overflow cap? Is there a air bleed for cooling system? Has done this with original tstat, no tstat and new tstat! Just put head gasket sealer in. Thanks. Ready to replace truck!!