I've uploaded a new prp3 that fixes a few minor bugs and avoids the problems with nonrandom data by doing specialbutslower multiplications for the last 25 iterations.
Let me know if you find any problems. 
I also downloaded and tested the new version of prp. For me, it is about 3 times faster! (19*2^531k+1; 32k FFT)
Congratulations, George! Just one thing seems somewhat strange. (k*2^exp+1) If I use k=19, I can go beyond exp=595000 and it still uses 32k FFT. If I use K=10000 or so, it switches already below exp=447000 from 32K FFT to 48K FFT. with k=3 exponents upt to 638000 are possible with 32k FFT. in the k*2^exp1 we have more FFT lengths: k=19 has 32kFFT up to 601000; k=3 up to 645000 and k= 10000 up to 453000. In the minus mode, more FFT sizes are available (28K and 40K instead of only 24K and 40K besides 32K). Is that normal, that the threshholds of the fft sizes depend so greatly on the size of the k (mantissa)? If we use k=3 instead of 10000, the the difference is just 13 bits. 
This is all normal. In my modifiedPercival IBDWT, log2(k)/2 bits are required in each FFT word.
So let's say the 32K FFT can handle 20 bits per FFT word. If k=1, n can be 32K*20 or 640K. If k = 8191, then log2(k)/2 = 6.5 bits. That leaves 13.5 bits per word. So now you can only handle n up to 32K*13.5 = 432K. 
Warning: A bug has been uncovered for k values around 7 to 9 digits. A fix is underway.

The fixed prp3 can now be downloaded.
You can get the versions from: Windows: [url]ftp://mersenne.org/gimps/prp3.zip[/url] Linux: [url]ftp://mersenne.org/gimps/prp3.tgz[/url] The linux version is untested, I do not have Linux running on any P4s here. 
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