Okay, so obviously something is loose, which is causing this. In other words, mesh feels good on the bench, but when it's go time there is a part that is either already (or becoming) loose and causing it. You probably know that...just setting base. ;-)
Here is what I would do: After it happens, treat it like a 'crime scene'. In other words, go through everything slowly and diligently. First, check your mesh after it happens. Is the mesh still good, or has it moved at all? If it's moved, then check the common problems (engine mount, bell play, spur gear, etc.).This will simply show you something has moved and you need to find piece by piece. What I do troubleshoot this is actually tighten down each piece. By doing this, (assuming everything is already tight prior) that is your offender.
If the mesh still seems good, then ensure that your flywheel straight. Often mesh feels good on the bench, but if clutchbell/spur gear are no perfectly aligned...bad things happen. I'd also check your spur gear and see how much 'wobble' you are getting from it. Roll it fast and see if things don't sound right. Start it up and drive slowly and to determine if you here 'clanking'. The good news here is that there is only a small number of things causing this. I know this is all basic information, but things need to be ruled out. How is your transmission? Anything burrs on the gearing?
