I would have had this easy from the buried alive part, but the bulk of the details are off.
Serious spoilers . . . Like the entire movie explained.
She gets tired of being away from the city, so when their attempt at pregnancy works, and she sneaks off to get an abortion, that's the doctor she has an affair with. Then they use one of his fish from his salt water tank, his idea, and she poisons him with it. She puts the poison from the fish in his wine. It makes it appear that he has a heart attack, and dies. . . .
Lion fish
Fortunately in their rush to get the hell out of town with all his money, they rush the funeral. When the funeral home sees that not much money will be coming in on this funeral, they use an old rotted coffin. They also don't bother fixing the embalming machine. Good thing for Tim!
He was actually going to kill them rather easily, by shooting them, but when he heard them talking about the baby, he decided on sweet revenge. When the doctor shows up to kill her with the same poison, so he can have all the money for himself, Tim Matheson locks them both in the basement. He turns the house into a huge maze. When they get to come up, they are lost in it. He shows himself, but with his face covered. He blocks her so that only the doctor can come forward. The doctor still has the needle of poison, and is going to use it on who he thinks is the sheriff. The sheriff knows something is seriously wrong. Which is confirmed for him once Tim Matheson's body disappears. Anyway, as the doctor gets close enough to see that it is Tim Matheson, he turns away to run, but falls on the needle. Oops! So he is taken to the end of the maze, which is a coffin box. In the end, he tricks her into crawling into it, by closing off all the walls of the maze, etc. He has her, the dead doctor, and all the money in the coffin. He then burns the house down. After that, he drives off with that pine coffin in his truck. He buries them where he was buried. Then the sheriff who was just at the fire, and had been told that there were no bodies, walks up behind him at the grave site and say, "Mr. I don't know who you are, or what you've done, but I don't ever want to see you around here again. You hear me?"
End of spoilers
Yes, I have and love this movie. Oddly enough, I've never watch
The Shawshank Redemption. Go figure!
p.s. There's a sequel! It stars
Tim Matheson, and
Alley Sheedy. I don't care for her acting, though.