The BKR7E-11 plugs should only be used and gapped small like you mentioned for a turbocharged application.

If you are not turbo, you should be running the 5E or 6E plugs. The correct number is BKR5E-11 or 6E-11. The 5E's are a range hotter and are used in NA applications. The 6E is the stock replacement.

Gap them to 0.048 - 0.050.

Aside from that, check for loose wiring/connections. These cars are old enough now where the wiring can become brittle and the termination points inside the connectors can start to become loose.