It could indeed be memory. According to Adobe, the older 32-bit version of Photoshop is capable of using up to 3.2 gb of RAM, the newer 64-bit version "as much as you can give." Try this (on a Mac--Windows procedure is similar): In Photoshop, open Preferences > Performance. Under "Memory Usage," move the slider to "let Photoshop use" 70 to 75% of available RAM (recommended for optimal performance). You can set your History States lower (e.g., 20 or 30) which will lessen use of RAM. Set Cache Levels to 4. You can also open the Mac's Activity Monitor (Applications > Utilities > Activity Monitor) and click on the "Memory" tab at the top of the screen. This will show you just how much memory each application and process is using at any given time. On my computer, with Photoshop open but inactive, it is consuming 3.06 gb of available memory. I'd suggest closing all other unneeded programs and applications before starting your work in PS, since some apps working in the background (notably web browsers) will spike their usages of RAM (you can see this in Activity Monitor) for no apparent reason. I am no computer expert, but I've worked on Macs for over 30 years. Since you say the stacking process sometimes works when you first open the program, what is likely happening is that when first opened PS is "clean" and using minimal RAM. Once you start working with it, the caches, history states, and other processes start to kick in utilizing progressively more RAM. If you bump up against the max, PS will punish you by not completing the process, crashing, or running glacially slow. I could be totally wrong, as your 32gb of memory should be more than adequate, but at least it doesn't cost anything (or screw up anything) to try adjusting PS preferences. Hope this might help--good luck.
