2002 vx ss 5.7l 260000km's

Problem;

15 min on the highway and the ticking of a lifter starts, only when i travel over 120kph.
Pull over for gas and when I restart it goes away, then 15min later it comes back!
If I don't pull over and shut down then other lifters join the club and it sounds like a box of spanners under the hood!

Ive tried thick oils and thin oils and the problem remains the same, I thought an engine flush might do the trick too but no!

Anybody got any ideas? I'd love an easy fix but the more I think about it the more it's looking like a head job!

Your help would be greatly appreciated as I live in Dubai and don't really want any of the non trained monkeys touching my baby!

Cheers lads