http://www.p-38online.com/p38j.html
In spite of all the advances and improvements, the P-38 J would suffer in
the European Theater of Operations. Lockheed did all the testing in the
United States, and were never tested in conditions similar to those in
Britain. Once the P-38 J arrived in Britain, problems were encountered
immediately. In addition to the new problems, many new, inexperienced pilots
were beginning to fly. The problems would take time to work out, but the 8th
Air Force was not able or willing to wait. At this time, P-51 D Mustangs and
new versions of the P-47 Thunderbolt were available to perform long-range
escort and strike missions. This was the beginning of the end of the P-38 in
the ETO.

-HJC