Antti,

Perhaps a electrical problem.

Turbo glowing red under low boost suggest massive retard of the ingnition to me (causing detonation in the exhaust manifold and would explain the popping/backfire in the exhaust..
Is the ignition timing correct? Could the distributor have moved?.
The heat could do this to your spark plugs.

Matt

Let us know what the problem was.
check out this link and follow through there links to there diagnostic page http://www.spark-plugs.co.uk/pages/t.../diagnosis.htm
It has diagrams of plug fouling and its cause.