On many cars, a common leak is the timing cover seal. This happens because the harmonic balancer sealing surface gets a groove worn into it that prevents a good seal. The cure is what is known as a "speedy sleeve." This is a very thin polished nickel sleeve that you press over the harmonic balancer sealing surface. It works very well.

If you get measurements, you might find the correct size speedy sleeve at an industrial supplier such as MSC.

Since the seal has been replaced multiple times I doubt that this could be the problem, but it is relatively easy for this seal to "fold under" when installing it. MB makes a special sleeve tool that can be used to slide the seal in place without it folding under. I don't own such a tool, but have had good success by simply running my fingernail around the seal as I push the cover rearward and just see that it does not fold.

Good luck,