Why am I getting engine flat spots on my Audi Cab 2.6 V6 once the engine has warmed up?

I have a 1994 Audi Cabriolet 2.6 V6 and when the engine had warmed up I get flat spots around 3000 and 4000 revs. Its fine if I rev then engine when stationary but when drive it happens. If the engine has just been started it is fine and flys through all the revs in all gears. I'm wondering if it may just need some fuel treatment or something?|||Most likely the mass air flow meter, located in the air intake, if fuel injected. Problem will only appear when engine has warmed up and the excess fuel device has cut out. Get the fault codes read, this will indicate if the MAF is faulty.|||Most likely to be down to the spark plugs or ignition leads breaking down under heat and load.