Does this particular symptom ring any bells?

Driving about 70mph, box is in 4th (overdrive) at about 3000rpm, then for no reason it changes to 3rd and revs obviously shoot up, then after about 2 secs it changes to 4th again. It keeps doing this about every 10 secs.

Fluid level appears to be ok and looks new. Ta.

you could try to adjust the tps on the right hand side of the engine. an incorrectly set one will throw out the box and could cause these symptoms

Is it happening when you are coming on and off the throttle ? if so you,ll find it,s the torque converter unlocking when you take your foot off the gas and locking back up a couple of seconds after you put your foot on the gas again , this happens in fourth gear .

If it,s happening on it,s own when your foots constantly on the gas then as above , check TPS setting . HTH

Thanks both. It does it when holding the throttle steady.

Done some searches on setting the tps so will have a go at that tomorrow.