This turned out to be the 3.5" silicone intake elbow collapsing right on 5000 rpm. The thing popps back out after i lift off the accelerator.
I have been chasing this fault for a while, and it suddenly hit me as to what it was. Can't believe I missed this before.
I had changed my S5 solinoid, trans fluid and filter, my trans inhibit switch, my speedo sender, added an MSD tach adapt and changing out my distributor along the way.
I also did all this after getting a code reader (too lazy to use a bridge and light any more) and sorting my faults out along the way.
Whilst fault finding I also exerimented with locked timing by disconnecting the SPOUT wire.
So a bit of a journey, but glad to have found the problem.
I have hose clams around the elbow to stop it collapsing until I can change it out, but it seems to work as i pretty much shat myself past 5k rpm.
Thanks to everyone who had input into this.
Much appreciated.