okay.. let's come back: did u backflush it ?? (usually at a lube place)
Or are u just literally flushing it ??
Is the T-stat okay ?? Are u sure ??? Maybe wrong direction ???
just make sure the t-stat is ok... and if u r only flushing, I would recommend a back-flush in a lube place... it costs only $30 and it really cleans the whole thing.. they use steam water.