dies at normal op temp please HELP

[port rover]

hi guys,a few weeks ago my 1995 (m) rover 214 16v (mpi)died after the radiator sprung a leak and fried my head gasket.i put in a new radiator and i got the head skimmed and my mate(garage owner)fitted a new gasket with the steel dowels.while bleeding the coolant it cut out just at its normal temp and wouldn,t start again until it cooled .it is all new sensors (crankshaft ,ait ,lambda etc)and i have had rover check it out and all tests passed.
its really scunnered me as it runs great until it hits idle and once you bring it to about 4000 rpm it just dies as if there is no power at all.is it the ecu ,when i open my doors (disable alarm)the led on the dash starts flashing rapidly as if it is a fault code or something,i know this is not normal as my wifes does not do this. if i wanted to try her ecu would i need her alarm unit aswell for the engine codes to start.

any help is much appreciated

ps i have a super chip on this car but disconnected it to test but it is still the same has this maybe altered something.i was told these do no damage just alter your timing