I agree with Artur. You're running into the same freeze-thaw issue that you would with any protein. Commercial polymerases are typically in glycerol which keep them liquid at -20°C, but definitely not at -80°C.

I would subaliquot the next time it's taken out of the -80°C for use into much smaller working volumes. Then when needed, you could just thaw the smaller volume and keep in a regular refrigerator where it would probably be stable for a week or two at least. 

Just make tiny alliquots of 5ul or so (whatever is suitable for your use) in PCR tubes. Throw them into a box, lable the box and only thaw them once-the time you use it.

Thank you all for your answers to my question. I appreciate your generosity with time and advice. I will do as you have told and keep the Taq in a regular freezer. 

Thank you once again......

Best,

Dilantha

Dear All,

Thank you once again for your useful comments........I will keep these mind when I use Taq polymerase.........

Ruwanthi, Hope you are enjoying your Ph.D. at University of Melbourne......I had a really nice time during my Ph.D. at Melbourne Uni - people were so helpful !

Best Wishes

Dilantha

This Temp. is  not good for the most enzymes including Taq polymerase. Its effect on Tertiary structure of enzyme that could affect its activity.   

Like Mr. Artur said, repeated freeze-thaw can lower the quality of polymerase. I suggest you to store it at -20 degree C if you want to use it on daily basis