1997 vauxhall vectra 2.5 cdx?

engine overheating. runs at 90c for about 12 miles ,then temp rises to 100c put heater on temp goes down to 90c. if used on a 7 or 8 mile trip left for a couple of hours , then drive 7 or 8 miles home temp goes up into red. fans seem to be working flat out .I' had engine plus rad flushed out plus new thermostat.
not losing any water no trace of blown head gasket any idea's