Variable engine write hangs

Question

I am using the shared variable engine to bind shared variables to an OPC server (DSC/Labview 8.5)
When using read only variables, all works OK
When using a read/write variable (specified either R/W or W only in the shared variable definition), the shared variable engine hangs, and needs to be stopped and restarted for the program to work again. If the engine is not stopped and restarted, I basically get a shared variable is not located error (same as if I stopped the service manually)
I have tried synchronous or asynch writing as per the OPC Client I/O Server Advanced tab definitions (as well as In or Out of process).
Any idea why writes would be failing, but reads are working OK

Answer 1

a timed loop fires say every 1 sec, so I may have up to 1 sec latency between the clicking of a switch, an the action required.
If I set the loop interval to less than 500 ms, then I get long delays between the writing to the variable, and when it actually occurs. Even shorter times, I may never see the change in the variable.
I don't need fast updates, I just need them to start occurring without a delay when the event happens. In addition, if possible to have some way to allow the next setting of the variable to happen when the first one is finished.
In effect the sequence required is
1) Write to the variable
2) dissallow any further writes to the variable
3) check if the variable state has changed to that expected, and reallow variable write access
4) have a safety check, so if the write does not happen after a given time, time out, and reallow variable writes