Why is my excel VBA code execution randomly slowing down?

There's still a few more things you can do: (i) Set application.enableevents = False This can make sometimes make a massive difference. Don't forget to set it back to True on every exit path from the procedure. (ii) If you have opened the VBE at any time for that instance of Excel it will run more slowly. Even if you close it again. Restart Excel to resolve. (iii) The most noticable performance gain (depending on the size of the array) is never to loop through an array and write to individual cells as you seem to be doing if you can help it: .cells(i,j) = array(i,j) If you're writing to more than a hundred or so cells in one procedure you should write to a variant array and then dump that in one fell swoop to the worksheet. It is MASSIVELY quicker - test it and see. Something like: Range(sh.Cells(1, 1), sh.Cells(UBound(myArray, 1) + 1, UBound(myArray, 2) + 1)) = myArray (iv) Memory leaks. I generally haven't noticed any in Excel (and I used to do a lot of VBA) but you can introduce them in your code if you don't explicitly set objects to Nothing in your procedures. Only (ii) and (iv) would explain the inconsistencies however. (i) would also, if you happen to have other AddIns loaded occasionally that might be firing up when you write to cells which will raise sheet change events. On preview - paulsc makes a good point.

This is probably a silly question that you researched a long time ago, but have you noticed a load on the machine at the times when the code is slow? Is it during an anti-virus check? Crap like that?

That's a fair question. I've checked in the past, as you said, but haven't checked recently. Currently my code is slugging by so I just checked the performance. EXCEL.EXE is taking up 50% of the CPU System Idle Process is taking up about 47-48%, with firefox and acrobat and a few other programs fighting over the last 2-3%.

The most common reason why performance of interpretive languages varies drastically is http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29. The easiest way to avoid it is to save your spreadsheet, terminate Excel, start it again, reload the spreadsheet, and then do your calculation. The memory heap should start clean.

I see that you've tried restarting Excel. The problem here is how to make the VB runtime libraries unload, and I'm not sure that's easily done.

Class Goat - You could write a native VB application that drove an Excel object. Terminating the application should unload the resources, assuming objects are properly handled.

"... We usually attribute it to 'the server being slow' but we notice similar issues even when files are local. ..." If it is really this random, I'd strongly suspect that some element of what you are doing is not local, and is, in fact, server dependent. I've seen things as weird as http://www.microsoft.com/windowsserver2003/techinfo/overview/scr.mspx of what users swore were local files cause big slowdowns on networked machines. I can't say definitively that such a thing is the root of your problem, but I would sure be looking hard at network traffic, in conjunction with CPU usage, when such slowdowns occur.