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 correctly connected.
I've owned the same model since new, nice car.