fit a vaccum guage in inlet man. and start up see if vaccum is good and steady about 21 inches of mercury ......let it run and if vac starts to drop and gauge has a pulsing effect , if so sounds like valves being held open .

2 possible causes fo this

As engine warms up and valves expand and they get stuck in guides especially if guides are laquered up.

Or if oil grade is to heavy hydralic buckets carnt releive themselves of there oil and they jack themselves out giving loss of compression.

The peugeot 206 cc petrol engines where prone for this in winter , using a 0 30w weight oil used to help alot , in this case it was the buckets getting a laquer build up that caused them to jack out when oil was cold.

HTH