460GLEi Automatic not running over 300rpm

Can anyone help in curing a slight problem we have with our 460.A few months ago whilst driving we went for kick-down to go up a hill. The box dropped down a gear but wouldnt go past 3000rpm??? You can apply full throttle and she makes a really restrained sound as if she's got a "rev limiter" fitted. We've been told its many things from air leaks into the manifold to not enough fluid in the gear box. We had recently (two days before) serviced the car at home before this started. If anyone has any suggestions on whats wrong and if possible how to rectify then we would be most grateful, the poor girl doesn't drive too well at the moment. Its a 1990 H reg 1700 with 98500 on the clock for info
Thanks in advance