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.
Dont forgetthe BHP quoted online will be at the Flywheel and not the wheels, in sure someone with more knowledge will be able to give you a rough idea of what the Flywheel BHP will be around with a Wheel BHP of 139.