Problem with 2X9800GX2 and cutWaitForThreads

I use 2X9800GX2 so four GPUs(windowsXP).
I launch four threads with similar length of calculation.
I can see the lenght for each GPU(see code below), I have all in less than 0.4sec.
Howewer the “cutWaitForThreads(threadID, nb_gpu);” waits always 0.9/1.1 secs, instead of something near 0.4 secs.

I have an another machine with 2 8800GTX and where I have not this problem.

Thanks in advance. :)

clock_t start= clock();

CUDA_SAFE_CALL( cudaThreadSynchronize() );

do_GPU(psio);//here is the call of the kernel and a CUDA_SAFE_CALL( cudaThreadSynchronize() ) at the end

clock_t end= clock();
psio->temps=(float)(end - start) / (CLOCKS_PER_SEC);result 0.35 sec/0.40sec :) 



main (…)

SINPUT_OUTPUT sinput_output[4]; // define a struct for each card.
CUTThread threadID[4];
int gpuIndex,nb_gpu;



clock_t begin = clock();

//Start CPU thread for each GPU

 	for(gpuIndex = 0; gpuIndex < nb_gpu; gpuIndex++)
        threadID[gpuIndex] = cutStartThread((CUT_THREADROUTINE)     GpuThread, &sinput_output[gpuIndex]);

  	cutWaitForThreads(threadID, nb_gpu);

clock_t end = clock();
printf("calculation in %f s\n",(float)(end - begin) / CLOCKS_PER_SEC);
            //result 0.90/1.1 sec!!! :(  :(  :( 


in fact, I made a mistake in my description of the problem:
The lenght can be very different beetwen the cards.
Sometime all “psio->temps=(float)(end - start) / (CLOCKS_PER_SEC)” in “static CUT_THREADPROC GpuThread(SINPUT_OUTPUT * psio)” are beetween 0.35 and 0.40s,
but some time the card number 4 makes the calculation in0.9/1 sec.
At the minimum card number 4 is 10% slower than card number 1.

Howewer all the cards do the same.
So my question is: why the lenght in the card 4 can be so volatile?
Why can I do to avoid that?