Approximately four months ago I changed the driver's side header on my Buick GN. Pulled the header and had the typical crack repaired. Replaced the header (with no apparent exhaust leak) and eventually over the next week realized there was a loss of power. My symptom is......the car runs fine at idle and while driving. Apparently gets plenty of boost (17-19 lbs.) but barely spins the tires like it once did. Hooked up to Powerlogger and all the vitals appear to be in normal operating range at idle and during driving operation. Initially thought it might be vacuum related. Checked all the hoses for cracks and used the starter fluid method to verify any leaks, with negative results. The next thing I though it could be was the torque converter; so I changed the torque converter to a known good one and the symptom remain. Also replaced the fuel pump and spark plugs with negative results. Looking to see if anyone has ever experienced anything like this and could offer any advice in chasing down this dilemma. ANY help or advice would be appreciated!......