O2's all over at WOT?? why?

[1985GN]

ok guys here is the story. with the chip i have FP should be set between 38-41psi line off. so at part throittle if i got on it a little not WOT o2's would choot up to about mid 800's which is nice. so at WOT with 38psi the o2's would vary from low 700's to 800's. sometimes i would get knock with the 02 too low. so i have the FP set at 41psi line off and now i get sometimes low 700's to mid 800's. why do they vary i have even gotten a few 600's sometimes. it seems to have a nice high 02 and no knock when it first starts up and as it gets warm the 02 and knock get a little more when hot but not always. it just jumps around alot sometimes i can have a nice run and sometoimes have to let out. what could be causing this?