Volvo S60 T5 (B5234T3) 2001 problems

HI,

I recently gave Upsolute a try with a modified ECU. I have a 2.5" catback system from Ferrita and now the mentioned Upsolute upgrade.First three gears feels much improved, so does the 4th and 5th. The problem I'm experiencing is that on 4th and 5th under full throttle I feel a little "jerkiness". It's hard to explain, but it feels like the engine is lacking some air, even though I doubt it really is. The feeling gets worse with more load (more passengers) and taking the foot of the pedal in 4th or 5th produces a "soft boom" from behind. It's not a misfire or explosion, but feels and sounds like a ball of air booming out of the exhaust.
All of the above is less feelable with 99 octane gasoline, but still there to a degree. 98 feels very rough under hard load.I think I've had some knocking/pinging, but I'm not really sure. Under full load with two passengers on 4th gear with 98 octane going upwards (yes, somewhat extreme conditions, but still!) the engine stalls (almost like antispin kicking in) and responds without any fireworks for the next 15-20 seconds or so. I believe this is due to the ECU retarding the ignition because of a "ping".
What can I do to remedy this? I realize I should not stress the car/engine under such extreme conditions but still... The possible routes I have in mind is to "detune" the ECU and retard the timing a few degrees and/or narrow the gap on the spark plugs. Another possible solution could be to change to a cooler heat range of spark plugs, but I'm not sure that is the correct approach.What do you guys think?

Any help will be apprecited.

I didn't find the right solution from the Internet.

References:
http://www.upsolute.com/forum/index.php?topic=10431.0
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Thank you.