Over revving

hello I am new to this so please forgive me if I posting wrong

I have disco 2 td5 2000 auto

When I reach around the 30 mark when it goes to change gear it's as if it gets stuck and over revs can get over 4000 rpm before it will change gear then all of a sudden it will come alive and throw me back into my seat, this also can happen at 50 but not always.

I have checked the maf sensor looks to be OK have cleaned it with maf cleaner the air filter box is missing a clip and looks to be loose but from reading other threads this is not an issue took it to my local garage he has changed a few fuses and sensors ( not sure what ones have got a receipt as was all under counter cash in hand

Have disconnected the maf sensor and had a run around made a bit of difference but not much to be fair.

He thinks it could be the gear box and would need to take it to a specialist to have a look but that could be a costly repair money I haven't got.

Has anyone had this problem with there's before could it be a simple fix a cheaper fix or is it my gear box

Any help would be great

Oh and I'm a complete novice so please keep it simple

Many thanks

Chris

First step would be to renew the gearbox oil and filter