The solution is removing all the break lines (BR) in the source code of the table. Each break line causes a shift downwards. You have to write tables (and layers) so that they consist in one single (and long !) line, if you want to get a good result.

Not good:
[table]
[tr]
[td]
blah
[/td]
[/tr]
[/table]

Good:
[table][tr][td]blabla[/td][/tr][/table]

A GuppYHTMLOptimizer utility is at your disposal in the Downloads. It eliminates all the visible and invisible jumps of line from the codes of tables.