If theres no pressure, it stays on 0.
Fine so far
When theres pressure, the value gradually( 2 seconds) goes up, then gradually goes down, stays on 0 for a few seconds, then goes up again (with the pressure being more or less constant), and so on
How do you read it?
Do you have access to a 'scope?
How did you connect it and what other components are used?
What piezo do you use?
A piezo doesn't take well to constant strain.
That said, what you should see, if the pressure change is sudden, is a short pulse ringing for a short while (nothing like 2 seconds, unless you build up the pressure gradually and the output is quite small), after which it will fall back to 0V output - it can only tell about relatively quick changes.
A piezo will output up to hundreds or even thousands of volts if unloaded or loaded with a high impedance and the physical change is very short.
You cannot use a piezo for measuring static pressure. Strain gauges would be a solution, or you can get a ready made pressure sensor.
On-Semi (formerly Motorola) makes a broad range of cheap pressure sensors, for absolute as well as differential pressure.
Im connecting it as indicated on the arduino site, although instead of a 220 ohm resistor, mine is 20k.
Why the change?
Even if it isnt possible to control how much pressure is being applied at all times, if I could just know when theres a significant variation of pressure, it would do.
That's all it can tell, but if the change is too slow, the generated voltage will be next to nothing.
So what IS the rate of change?
If you reveal what you want it to do, I'm sure we can find you a better method.