Help!!! FreeSpaceMap hashtalble out of memory.

Cygwin, 7.3.4
This thing is really KILLING us and our customers.

In pgerr.log this always go together:

WARNING: ShmemAlloc: out of memory
ERROR: FreeSpaceMap hashtable out of memory

Theses errors usually take place on INSERT statements like this one:

INSERT INTO params (param_id,map_id,param_key,param_value) VALUES
(0,858,'MAP_NAME','New Map')

params is just a simple table (no triggers or rules involved):
CREATE TABLE map_11.params
(
param_id int4 NOT NULL,
map_id int4 NOT NULL,
param_key varchar(1024),
param_value text,
CONSTRAINT params_pkey PRIMARY KEY (param_id)
) WITH OIDS;

The db server machine has 4GB of physical memory and more than half is
always available.
Number of connection is 64
Number of buffers was increased to 81920 (but this does not help)

Is there any parameters that can be configured to increase available shared
memory size?

Any help is greatly appreciated.