Have a 4.7FD with LSD from UKDM DC2 (exact same box as 98spec Teg) full gearbox but has a cruching 3rd gear, so its sat in garage as i replaced full box. Would be perfect to strip for parts.
I had this when i had a Spoon ECU (JDM) on a UKDM DC2 (B18C6). From what i recall it is to do with the IACV being different within the ECU's.
I changed back to a UKDM ECU and it was fixed.
Id be looking into where the coolant has gone, as it should be a sealed system. Check for leaks, hopefully it is that as worse casr it could be the head gasket.