There's no mistaking spark blowout. The car misses and stumbles like crazy, you'll feel it too as the car shudders from missing.
Clutch slip usually has the RPMS climbing rapidly as though you're pressing on the clutch pedal. Look for RPMS and MPH suddenly going out of phase for clutch slip.
Like others have said it may be fuel or something else. Pegging your MAF meter maybe? Have your tuner look over data logs (hopefully he took plenty) and check for injectors, pumps, maf pegging. Something was obviously happening.