Vauxhall Astra flat spot when starting from low rev

When starting off in 1st gear for example the car will hit a double flat spot until it rises above 1500RPM this would also happen if you try to pick up speed in other gears from below that rev range. The MAF has been checked on another car and is working fine. The car is fine at all other times when the revs go beyond 1500 and so drives quite normally. This is not a big problem but an anoying one that I would like to fix.