If the power drop was sudden BUT the engine still starts easily then the
valves are NOT the cause. Not to say that a valve adjustment isn't
needed but it would not show such symptoms.
By power drop I suppose you mean poor acceleration. If that's the case
here are some possibilities, in order of likelihood:
When were the two fuel filters last changed?
The engine's air filter last changed?
Clogged banjo fitting or cracked hose. The turbo's boost is piped over
to a fuel enrichment device inside the injection pump. This is done by a
plastic line that starts at the aft end of the intake manifold. There is
located the "banjo fitting" and hollow bolt. Remove the both and clean
out the bolt and the banjo fitting with a paper clip wire. Then follow
that line over to the left side fire wall to the "switchover valve" . Be
sure the rubber connections to that valve are intact. Then follow the
plastic line to the injection pump again ensuring that its intact and
I've owned the same model since new, nice car.