One thing we always do when measuring the activity of NADH/NADPH-dependent enzymes is to experimentally check the actual concentration of NADH/NADPH in stock solutions. Given the molar absorptivity (ɛ) of NADH/NADPH at 340 nm of 6.22 mM-1 cm-1, you can easily check it by simply reading a known dilution at 340 nm. NADH and NADPH oxidize easily even as powder. I think it is worth checking the concentration of a freshly prepared stock solution. Keep in mind that molar absorptivity also depends on solvent and pH.

I am not sure about the salt (e.g., tetrasodium salt or tetra(cyclohexylammonium) salt) you used to prepare the NADPH stock, but 80 µg/mL (~96 µM) should result in an absorbance of ~ 0.6 (after correcting for the absorbance of the buffer and cells). This means that in the first attempt ~65% of all available NADPH was consumed. Note that enzyme activity depends on substrate concentration, thus, with the same amount of enzyme you will observe a lower activity when the substrate concentration declines significantly (e.g., 65%) over time.