Our Imam always made mistakes in Holy Quran (Reads the verses totally incorrect) during Maghrib prayer,we ask him many times not to do Imamat and let someone else do the same (A 60+ age Hafiz Quran is always available in the mosque)but he never let any one other to come forward for Imamat.
Please let us know should we still offer prayer behind such person or it is better to offer the prayer separatley.we can not go to other mosque becasue it is the only mosque in our building and other mosques are too far so we can not go their during office timings.
If the Imam that you have described in your question makes blatant errors in recitation of Qur'an then you should have him replaced.
As far as the prayers are concerned, your prayers will be considered valid as long as the Imam did not make such major mistakes in recitation that would invalidate the Salah. If such mistakes in recitation were made with correction then the Salah would have to be repeated.
It is better to pray in congregation as oppose to praying individually unless the Imam makes major mistakes that invalidate the Salah. In such case it would be better to pray alone.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America