The O2 signal oscillates normally, it's not the easiest thing to test for 100% accuracy.
Check your ground strap and check the gauge wiring. Personally I'd run a tap trough the O2 bung and clean out the threads for a good connection... they should've known better then to just spray over that.
Ditch the Chiltons.
TSRM: http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=FI&P=47
I don't know if it would make the differences you're seeing, but you might also want to test for an offset between E1 at the ECU and your ground strap.
Check your ground strap and check the gauge wiring. Personally I'd run a tap trough the O2 bung and clean out the threads for a good connection... they should've known better then to just spray over that.
Ditch the Chiltons.
TSRM: http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=FI&P=47
I don't know if it would make the differences you're seeing, but you might also want to test for an offset between E1 at the ECU and your ground strap.