Don't worry about the fuel filter not being full, mine always looks half empty. I guess we will never know the exact cause, these old landies can have several things wrong with them and they'll still run so your mechanic probably found and fixed half a dozen of such problems and may not know which of them was the culprit. It's always a good idea to fix one thing then test drive it to see if that solved the problem, if not fix the next thing and test it again and so on. I had a similar issue suddenly appear, the engine would splutter and almost conk out. It was the contact points in the distributer, a simple adjustment and it was good again but it helps if you know what to look for when the problem arises. It sounds like your mechanic was reasonably priced, it will pay you to keep in his good books.
Col