what could it be?

ok so i have a problem of lean misfire from 1000-1700 rpms. anyway i replaced tps, temp sensor, maf, etc etc. so finally yesterday my mechanic hooked up a testor to the computer to test my 02 sensors and sure enough one of them was fluctuating (like it should be) but the other one wasnt. so i replaced both.....now the one that wasnt fluctuating IS fluctuating, but not as much as it should be.....not as much as the other one is......what could this mean? what else could it be? thanks

yeah, also, my mechanic said to check compression in my OLD ASS.....motor cause i already know ihave oil burning problems....and i put rebuilt e7's on there...he says upping the compression when i put those on could cause even more wear on the rings, causing what is going on.... is that TRUE?

basically no compression = lean?....somehow?