When you flush the toilet does it have a slow flush, or fairly fast?
It should finish flushing before you get your hands wet at the sink. If not then it is slow.
Ok what is most likely happening is the venting system is plugged partially, or completly at some point in the system. When another plumbing system componant drains water it causes a vacuume and pulls some of the water from the toilet.
To test this you can do a flood test and watch the toilet for signs of the water being pulled from the bowl.
To do a flood test all you have to do is plug off the drain of the componant you are checking, and fill it up with water. As you observe the water in the bowl have someone release the water into the drain.
Once you find the componant that pulls the water level in the toilet bowl down you can then determine which vent stack is the one that is plugged.
This will require someone that knows how these are configured and routed during the rough in of the houses plumbing system.
Once you know which vent stack is suspect you can go on the roof with a garden hose and run it down the stack and turn it on. This should flush teh obstruction down the sewer. If it doesnt then a plumbers power snake will be needed. If you are not familliar with how to operate a power snake then I strongly suggest that you not attempt to use one yourself. Retriveing a stuck power snake is a very expensive job.
Please post any other concerns here. Best, THS