[shorod]

It's a bit of a long shot, but since in reverse the gearing is fixed, it might be loading the engine just enough to induce a misfire that you don't really notice in forward gears. Since you do notice a bit of a rough idle, I would still recommend starting with new spark plugs and spark plug wires, checking and/or replacing the air filter, and cleaning the MAF sensor. Also, carefully inspect the vacuum hoses and fittings to see if any of them are cracked or loose.

-Rod