Please provide more information, like the version. Does the same thing happen when you try using Demodata? The problem could be that the definition of the record in the ddfs does not match the true physical length of the record. You can get the physical length of the record by right clicking on the table and getting the properties.
To get the id number of the table do a:
select xf$id from x$file where xf$name = 'tablename'; ----use the value you get in the next query
select * from x$field where xe$file = valuefromqueryabove and xe$datatype < 227 order by xe$offset;
Below is the query for the table class from demodata which happens to be table id number 12 in the ddfs.
select
* from "X$Field" where xe$file= 12 and xe$datatype < 227
order
by xe$offset;
Examine the biggest offset and size, when you add the two values, they should equal the record size that you got when you did the properties. Should they not match, you will have to modify the definition using an alter table to add a column,lengthen or shorten a column. Please refer to the documentation and please back your ddfs prior to using the alter table.
You may consider opening a Support ticket if you need further assistance.
Jose