i know this thread is old but i just thought i would add what i have found with this problem for anyone who does a search on the topic,
i had a very similar problem in a rb26 i had the hed presure tested at one shop they said it was fine replaced the block and did a full bottom end rebuild and still had a problem with water oil in the water! i bypassed the oil cooler and still the same. i pulled the head back off and took it to a different place to have pressure tested, now here is the thing?
when the head was cold it pressure tested fine however they boiled the head in a oil bath got it upto operating temp and re tested it hot and it showed a leak between cylinders 3 and 4 in between the water gallery and the and the oil gallery that feeds the vavlespring buckets. It was unable to be seen visually.
Make sure when you get the head checked you go to a well reputed shop that has the facility to pressure test it hot and cold
cheers