It seems like the engine in my 3.8 E gets up to about 4k and kind of bogs down a bit…not that it stops revving just seems to struggle above that…not that I go there often… ….??
Two things to check: total advance at hi rpm, and is it going lean.
I have read before on Jag-Lovers that a failing coil can cause problems at higher RPMs. I have not experienced this myself but thought it was worth mentioning.
Seems like my total advance was about 36 last time I checked…
My E-type would do this on long uphill motorways/under load.
U0 needles sorted it, esp with foam filters, if I remember which I think it has (not here with me today to check).