Since you have already had the vet check the cat then it is probably safe to assume that the problem is behavioral. I would suspect that it is indeed your being gone for such a long period of time. Here are a few things to try:

Can you take steps to retrain the cat to the box. Basically you need to confine the cat to a room like a bathroom, or a large cage with only food, water, bedding and the litterbox. As he begins to use the litterbox consistantly you can expand his space. There is more information at this link:
Feline Behavior

Since you have been gone for such lengthy periods, Lumpy may be pooping and peeing outside the box because they've gotten too dirty for him to use. Some people have great success with self-cleaning boxes.

You need to thoroughly deodorize wherever the cat has been peeing and pooping. Petastic is highly recommended as is Heaven's Own. It is the smell of the "accidents" that causes a cat to reuse the place.

Check into using Feliway, a spray that mimics the happy scent cats leave when they rub their check against something. You can get it from your vet or from Cat Faeries at http://www.catfaeries.com
If you visit the website, you will see a phone number that you can call and for a fee discuss your cat's problems with someone. The person who does this is very knowlegeable and it may be worth the $$ for you. Feliway is also available in most large pet supply stores. There is also a plug-in with Feliway that can help, although they are pricey.

Finally, if all else fails, ask your vet about using an antianxiety medication such as Buspar or Prozac. These very often work, however the decision to prescribe them will rest solely with your veterinarian who will be able to monitor the dosage and side effects. In Lumpy's case this may be the best option because he is more than likely acting out on being lonely.

Good luck! Let us know what happens!
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Smile, it will make them wonder what you are up to!

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