A question about handling large-scale data

Hello All Nonmem Users, I am modeling intra-gastric H+ concentrations as PD biomarker, which varies from 10e-7 to 10e-1. I log-transformed original data and used "Y = LOG(IPRED)+EPS(1)" as log-linear error model firstly. The profile could be simulated well, but when I fitted data, error messages as "rounding error ." or "numerical difficulty ." showed up. Fitting was terminated generally. Will anybody share their experiences or tips on this kind of data? MANY THANKS IN ADVANCE! Cheers, Dongyang Liu, Clinical Pharmacologist Phase I Unit, Clinical Pharmacology Research Center, Peking Union Medical College Hospital, Beijing, China M.P.: +86-18610966092 O.P.: +86-10-69158356