Could you be a bit more specific? What is the solvent you suppressed, water? What was your lock solvent? (Addition of such an amount of H2SO4 will cause enormous downfield shift of your OH peak).

Hi, Imre. Thanks for your reply. The solvent I'm suppressing is water and I added a little D2O to do the locking. 

Excuse me Mengwei for that I haven't noticed your reply. I think your problem is mainly due to H/D exchange, i.e. that you are making deuterated sulfuric acid and may be also to that your lock frequency changes. Instead of adding a little bit of D2O to the solution, in your case I would use an internal capillary filled with a different D-rich solvent (d6-DMSO, d6-acetone, d6-benzene, etc) to lock. I am curious whether this will solve your problem. I suppose your are analyzing the compounds dissolved in your diluted MeOH solution, aren't you? Can't you avoid dilution of your MeOH sample?