help with slow toilet problem

I have three toilets on two floors.
My wife's the one who uses the master bath toilet most. It's upstairs and farthest from the main sewer line.
The main bathroom is also upstairs and it often smells, but I don't think its a sewage problem, but a problem with aim... we have two boys under 10 years old...
The downstairs bathroom is above ground in a third bedroom.
We have a septic system that was installed three years ago (August 2006) and last pumped August 2009.

When the upstairs toilets are flushed, the downstairs one gurgles. The upstairs toilets usually flush ok, but occasionally go slow. The downstairs toilet often flushes slowly - 5-10 minutes for water level to lower from rim to lowest (normal) level. It has backed up (filling with sewage) a few times and overflowed once (flapper got stuck open, clean water filled the bathroom and the attached bedroom-replaced the tank innards with $15 replacements from Home Depot).

Here's the kicker... it does not back up, gurgle or misbehave in any way when the dishes are being washed manually or with a dishwasher (upstairs), folks are taking showers (upstairs) or the washing machine is used (lower level).

Where's my clog? Is it something I might address? Do I likely need professional help? A plumber or a septic specialist?

I would call a plumber to snake the main line first. If the line is clear then it is septic.

John

A clogged drain can seldom be cleared by a homeowner. The diameter and length of the auger is such that even if you can find the equipment to rent, it is too dangerous for a novice to use. This is one time you need to call on a professional. Don't even think about chemicals.

OK folks, we all missed an easy one (and it cost me $120 for the Rooterman Service Call)...

It was the filter on the Septic Tank ( I've got two tanks and there's a filter between the two.)

Oh, and my wife has long hair!!