Find not working on Hosted file

Hi, not sure this is the correct forum

we have a calc field for our Invoice total ... Sum(inv_invitm_InvoiceID::Total Sell Price) + (Amount Imported/1.1) ... which is the sum of all the invoice items records + the value of the invoice total for all the old invoices imported prior to starting with FM

Our file is hosted in Brisbane with nightly backups to my office in Sydney

So all works fine then one day doing a find on the cacl field only finds those records where the search value = Amount Imported .. it won't find any invoices where the Sum of the invoice items = the search value 

However, the Find still works correctly on the overnight backup files !!

the hosting mob reboot their server and it's fine again .. for a while then it stops working again

anyone ever come across this ?  the hosting mobs can't work it out  .. they host other company's files and haven't had any reported probs ? .. so far it only seems to be a prob with that field, although most of the time our finds are not on calc fields

thanks

Cate