A bit of a long shot, but since you replaced the head gasket did you remember to put FIPG under the front cam caps per TSRM? They will leak without it..
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=EM&P=52
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=EM&P=52