The V-6 PRV engine loses oil delivery (volume) to the head over time due to blockage in the timing cover area. The rockers and cam lobes wear off quickly at that point. Replacing the cams and rockers doesn't fix the problem of poor oil distribution. The trick of removing the cams through the firewall (cam bearing bores are stepped bigger to the rear!) is a VOLVO dealer advised repair method. I have seen lots of cars with plugs glued over the firewall holes.
There is no cheap repair of these engines - they are a poor design in terms of valve train. Big tough cranks, great ports, stout rods and pistons. And it all means nothing because you can't keep cams, rockers, and chains in them.
Replace the engine with a four (big job, little upside), or sell the car. Never buy a V-6 PRV again.
And by the way, Volvo had no part in the design, only Renault and Peugeot.
I have rebuilt a couple for customers who didn't believe me. They run fine until they miss a few oil changes. At this age, nobody wants them rebuilt anymore, which is fine by me.