A condition can occur where the FSUIPC4.ini becomes corrupted that will cause the PMDG 737NGX to climb as though it is lacking power or fly with a Negative AOA or attitude indicated on the PFD. To correct the issue, look for an entry in the FSUIPC4.INI file under the JOYSTICKCALIBRATION section of the file for Flaps that looks like "Flaps=0,16380". If you find this entry, simply delete it and save the file and see if the fault is corrected.
If the fault continues, simply delete the FSUIPC4.ini file from the FSX\Modules folder while FSX is shut down and allow it to regenerate and the problem should be corrected. If you continue to have trouble after that, please contact us via the Support Portal for further assistance.