I think it's a Bitter SC.
From the net:
The SC was based on Opel's biggest contemporary model, this time on the Opel Senator, and was sold from 1979-1989 as a coupe, sedan and convertible. The SC was powered by a fuel-injected Opel 3.0 l-I6 (177 hp) or a bored-out 3.9 l-I6 that developed 207 hp (154 kW). Body design seems to have been heavily influenced by Ferrari`s 400i.
The first SC model to appear was the Coupe (1979), followed by the Convertible (1982) and the Sedan (1984). Production lasted until 1989 with 461 Coupes, 22 Convertibles and only 5 Sedans built.