Hey there sivfive I just got in...sorry I didn't see this post earlier.
Okay, so you have some large amalgam fillings. Unfortunately with all fillings, they don't ever have a perfect seal. So eventually with time it is normal to have decay under them. As far as options to fix, you mentioned the tooth colored fillings. Under certain circumstances they are a good alternative, but in your case from the way you describe it they probably aren't. For one thing, since you describe your amalgams as "large" the composite (tooth colored) isn't the way to go. They have a lot of sensitivity and really aren't the best treatment, especially in the back where you put a lot of pressure when you bite.
The second option you mentioned is probably an onlay. There aren't as many done now a days but they are great alternative, I've seen many last a very long time. If your dentist has experience doing them it is certainly something to consider. They are a little more conservative than crowns.
Now as far as crowns go, I'm sorry you've had bad experiences in the past. Unfortunately, those temporaries are exactly that...temporary. I too have had patients crack them and need to come in to have them replaced. It's hard to say without seeing an x-ray and the teeth, but just from your description (large amalgam fillings with sensitivity and decay under them) it is probably your best option. It's quite reasonable to expect decay under your fillings. Basically, you can look at it like this. You have large fillings now, and when your dentist removes the decay under them they will get a little bigger. Eventually, there will be so little tooth left that it will end up breaking on you. Do you want it to crack when you are on vacation far from a dentist, or do you want to prevent that now?
It sounds to me that crowns would be a good option in this case because your fillings are large and have even more decay under them. Did this present dentist do the temporary on the last crown that fell off? If not, make sure to tell them that. In cases like yours, I would make the temporary crown so your bite on them is very very light to prevent you from putting too much stress on them.
Let me know if you have any other questions or if this didn't make sense. Just from your original description though, the 1st thought in my mind was crown so I would trust that opinion (without the aid of xrays etc).
Hope this helped.
dave