Latency during send

I'm working on an app using OpenJMS to send a bunch of messages all at once 
and I'm experiencing some bizarre intermittent latency. Last week the app 
started slowing down and leaving a ~20 second gap while it sent 24 messages. 
We messed with various things, took some things up and down, restarted the 
machine and magically the latency disappeared. It worked fine all last week 
but yesterday the latency came back. At first it was 8 seconds, then 12 then 
16 or so. At this point it's taking about 500-600 ms to send a message. 
We've eliminated all the extraneous code and tested the timing with a simple 
message sending class. The timing is consistent. We tried moving the code to 
a faster machine but the send still took at least 100 ms. I searched the 
archives of the forum and found a few posts on latency but not a lot comes 
up in the way of answers as to what can cause it. Several people has the 
same problem I am having, though. That's not consistent with the estimates I 
read that indicate that OpenJMS can send hundreds of messages per second.

At this point we're considering restructuring the app to send all the data 
it needs to send in one large conglomerate message rather than 24 or so 
small ones. It seems that that might work faster. Does anyone agree? Does 
anyone have any thoughts as to what can cause intermittent latency that 
worsens over time like that? We've tried to eliminate potential problems in 
the operating environment one at a time but nothing seems to change 
anything.

Baffled. Any thoughts are much appreciated.

Thanks.