I drove my Scoob to work for the first time in a few months and noticed weird behaviour.

My route is basically, bit of urban, followed by 30 miles of motorway then another bit of urban. The car is fine for the first bit of urban, fine on the motorway, but on the second bit of urban it hesitates on light throttle openings off boost. Give it more than half throttle or above 3500rpm it's fine (apart from the odd loud backfire).

Car is MY93 WRX with Scoobysport system from the turbo back, HKS Induction kit and bleed valve (currently closed)

 

Take this : clean the MAF sensor, then use your original airbox when driving in wet conditions, or, the MAF sensor will get wet because the HKS let too much water trough its element...

 

Unfortunately, I don't have the original airbox, the car came with the HKS fitted.

 

Did you ever find out what was wrong? Mine does exactly the same bloody thing on a MY98

 

It was a mixture of boost related issues. The de-cat made the turbo spool up quicker, the boost solenoid was bit slow to control the spike resulting in fuel cut to control the boost. I now use a Turbosmart boost controller (similar to a Dawes) and boost control is significantly better. Cleaning the boost solenoid may help (disconnect the pipes and spray brake cleaner in there then reconnect and go for a thrash) it helped mine a bit, but din't clear it completely. It's not helped by the fact that boost cut on my MY93 is woefuly low (1 bar).

 

Just got mine back from SAS in Aldershot. I have had the Lambda sensor replaced.
Apparently the mixture was very very rich at low revs or pre-boost. Further up the rev range she weakened off to acceptable levels thus no real lack of performance.
The ECU showed no fault code as the sensor was still giving out a reading, so it thought everything was fine.
First impressions are a smoother drive lower down. Which I must admit was something very welcomed as I hated the driveability around that end. Time will tell if she is cured.
I assume that your problem has gone completely?