One for the gearbox experts

Got a problem with top gear on my 2001 4.6.
Very occasionallly it will refuse to change up into top.
First tme it happened i started driving and got the dreaded three beeps twice, but nothing on the message centre and the car seemed to be driving ok until i got to an open bit of road and i got up to the speed it would normaly change up, but it just stayed in 3rd whatever speed i got to, or if i eased off. Pulled over and stopped and switched off , pulled away again and it all changed up ok. most of the time it is fine but i have had this happen a few times in the last couple of days.
Gearbox fluid and filter were changed about 6 months ago and fluiid level was checked a couple of weeks back and was fine.
Is this likely to be an electrical fault or an i looking at an expernsive mechanical gearbox fault??
ANy advice appreciated

auto box consists of 3 parts electrics that decide when to change and what gear to change to ,valve block and solenoids that enable that and clutch packs ,oneway clutches that physically give gears ,check oil for blackish colour as this would show lock up clutch burning out ,get someone who knows how to use diagnostic test kit,check solenoids

So its a trip to my localy indy then. Can i check any of the wiring or solenoids myself?