I had a couple random stumbles, usually on the highway doing 3400rpm in 5th.
It was so sporadic I couldn't figure it out, but it was definitely an engine stumble not a mechanical issue.
After I changed the plugs (because it was time, not because I was troubleshooting this issue) I never had the stumble again.
Are you absolutely sure this is a transmission issue? I suspect it would be a fairly noticeable clunk if it was gearing. An engine stumble feels more like a hiccup.