Cant (yet) speak for a GT, but in my OB the hesitation showed up in low gears as almost no power if under 2k rpm - it would slowly accelerate until it reached 2k, then pull away as normal. Initially I was running unleaded, dealer blamed my driving style, service manager drove it even more conservatively that I do and said it should behave ok in that style. Once when I had the problem it cleared within a couple of 100m after adding the fuel treatment stuff to a full tank. I changed to 98oct and it went away for a year, returned briefly, I fitted the rubber washer under the knock sensor, problem went away, removed the washer after a few months, remained OK. It seemed to come in after several days of stop-start heavy traffic coming into city. You might blame somebody's driving if they change up a gear too early, but when it occurs taking off in first...
jp 05 OB 5mt