Slipping Auto Box

Hi, I'm hoping someone can advise me on this. I've seen similar threads but nothing exactly like my problem. I''ve got a 2.5 dse auto with 140k miles. I've changed the gearbox oil and filter about 15k miles ago. Very recently I've started having issues with nothing happening when I select either 1,2,3,D or R. Then after about 5 seconds it engages and drives fine. This developed into not switching up into 4th at 50 mph. It held in third with high revs even up to 65mph. Then after 10 mins of driving it worked perfectly. Finally a few days ago I was traveling at about 55mph, just entering the motorway and accelerating firmly. I lost drive, the revs shot up and I had to pull onto hard shoulder, I sat there for 30 seconds and it re engaged and I pulled off.
I'm planning to change the oil and filter again and throw a few litres of Lucas in but was wondering this is looking terminal. Any help appreciated. No faults showing whatsoever.

Sorry, it's a p38 2000year.

Have you checked the oil level ? there might be a leak
If the oil level is OK then it could be the pump or one of the clutch packs burnt out then you will smell it when you drain the oil it will be brown not red and clear

Oil and filter with some Lucas is the first step, make sure you follow the fill procedure, fill until oil runs out of the filler, then engine cold & idling, move lever slowly through all gear positions and back to P or N and continue filling until the oil runs out of the filler, fit plug and stop engine. Does the dash indicator match the lever position?