2001 vauxhall zafira 16 club stalling problem?

hi, i have a 2001 vauxhall zafira 16 club, it has allways used oil and would cut out when low, (no warning lights) top up and would start again. reciently on a hot day, cut out and would not start again for 1/2hr. got 4 miles and cut out again etc, rac checked no faults from diagnostics but changed crank sensor... same. i thought its a overheating prob? found squeezing rad. pipes it ran so thought i had cured it, but no it still stalled. found would start again if disconnected temp. gauge on thermo housing, noticed since squeezing hoses coolant light on but all full, any ideas? thermo seems to be working as hose at thermo end gets hot then cools, other side of rad. always cold, good fans or rad. blocked? thanks mark