Re: Flat Spots
I doubt it would be the lambda probe as all older lambda probes only work in closed loop feedback. ie they only work at constant throttle positions to alter fueling for fuel economy.
The TS engines have always been flat between 3000 and 3500 until the variable timing kicks in. This would be more noticable in fourth as your in that rev range for the longest.
Although it does sound like a fault I had with mine, it was more like a misfire. We remapped the car and sorted out all the fuel mixtures and the problem disappeared. Problem with this method is your only really masking the problem. Spending some time with the diagnostic checker I traced the fault to the Mass Air Flow sensor/Air capacity meter. Although the unit never showed up as a fault it was sending the wrong mixtures to the engine. This is an expensive route as they don't come cheap.
Couple of questions
Do you have a performance air filter even if it's a replacement panel filter?
Are noticing a loss in fuel consumption?
Having said all this as has already been pointed out the water temp sensor also has some control over the mixtures. Speaking to a lot of people the problem component tends to be the MAF sensor and not the water temp sensor.
If it's not a fueling problem it could be an ignition problem and the list goes on. Has the cambelt been changed recently?