What does the Stderr say? (The link in your first post is invalid for me).

I wouldn't expect any useful log information from an Invalid WU, since if something went wrong during the processing in a way that actually produced a log entry, the WU would likely be logged as "Error" instead.

"Invalid" is BOINC's way of saying, "You've finished processing the work, but the results you reported don't match with the results that others reported."

While this could potentially be caused by a software issue that leads an application to produce different results when run on different platforms/hardware (and I don't think WCG makes computer information available in the way that other projects do, so that would be difficult for users to troubleshoot), the more likely explanation is that your hardware is not entirely stable.

I think cliviafreak is right because sometimes I get a valid, and sometimes I don't. It depends on who I'm matched with. I hoped in posting this, that the techs would be aware of the issue and let me know if I should stop doing ARP or if it is ok.

I admit, yes, if the hardware fully complies and strict mode is used at all levels from compiler to hardware, then the results shouldn't change. To turn on that strictness severely limits the performance of software.

The reality, though, is that strictness is almost never fully adhered to and different hardware tries its best to comply. It never fully does. Further, compiler writers make mistakes. There are too many variables on the way from code->machine->execution to ensure floating-point computation is the same across all hardware and compilers.

And people want the performance! Even scientists running important simulations. If it's so important that a few decimal places matter, they use arbitrary precision numbers.

I'm not dismissing the possibility there is a fault in Unixchick's hardware, just that it is not the most likely explanation.