Since it is intermittant it is probably not the clutch itself or the transmission itself. I suspect a problem with the clutch hydraulicsm either the master cylinder (not to difficult to replace) or the slave cylinder (requires pulling the trans to replace). Shopuld you pull the trans to do the slave, you should probably do the clutch as well since you have already done most of the labor.
At this point you really need to show it to the dealer service manager or shop foreman when it is acting up. See if they will agree to road test with you immediately if you come in with it acting up then get to them as quickly as possible when it is acting up. Once they feel it they will have a better idea of what it needs.
