I have a macro that is reading data sequentially from 150 text files.
Sometimes the entire process takes 15-20 seconds, and other times it
takes a couple of minutes. I can't figure out why there is such a
time variance in the amount of time it takes to run.

Here are the main parts of the process:

1. Declare variables including dynamic arrays.
2. Size the arrays to 1000 elements using ReDim.
3. Create new Excel workbook.
4. Open first text file and read values into the arrays. Usually
there are 100 values per array.
5. Resize the arrays to 100 elements (or however many are necessary,
always much less than 1000) using ReDim.
6. Close the text file.
7. Perform some computations.
8. Set Application.ScreenUpdating = False
9. Write computed values to the workbook opened in step 3.
10. Format cells.
11. Loop through steps 4 to 10 until all text files have been
processed.
12. A little more formatting.
13. Save workbook and end macro.

If you have any suggestions, please let me know. Thanks.