Postgresql problem with update double precision

Hello ppl,
for few years I have problem when update double precision field. I have
table and few double precision columns, here is example:

sumall double precision,
sumin double precision,

My php script do:

$get = 2.40

and sql code is:

UPDATE table1 SET sumall = sumall + $get WHERE id = 1 AND rd =
CURRENT_DATE;

When I browse the table some times i see incorrect values like:

955.599999999998

it's should be 955.60 after these updates ... some days is fine, some
days the value is incorrect.

I have this problem from version 7 of postgresql, Im now use 9.0.3

Anyone know what can be the problem and why some times records is fine,
some times isnt ?

--
Regards,
Condor