2000 Vauxhall Vectra auto transmission faulty

Car I just bought, it goes really well through all gears when cold and after it heats up after say 20 miles or so the computer comes up with "automatic gearbox" and seems to switch the automatic shift system off. I then select gears manually, if I stop and switch ignition off and on again it will travel fine through the gears in automatic mode but resorts back to me making manual selection within a minute or so.
I have had a diagnostic test done and they said it wasn't repairable the codes that come up were PO715 input turbine speed sensor fault----next one was PO773 shift solenoid E Electrical.
I have changed the transmission fluid but no difference, old fluid had very, very slight tinge of yellow at bottom of basin I used, no metal shavings were to be found in old oil.
Do you think its a possible repair job without removing transmission your help advice would be much appreciated.
Thanks Alan