"During the time when the former USSR could afford to pump money into it, North Korea was doing pretty well".
The USSR didn't end in 1970. Besides, NK was not "doing well" then compared to now. There economy has continued to grow, and they are doing better economically than many other Asian countries, and better than most African countries. It is only compared to the spectacular growth of South Korea, that NK looks shabby.
The USSR never considered NK to be a reliable ally, and after 1953, gave them very little aid. Kim Il Sung (grandfather of the current ruler) had served as an officer in the Soviet Red Army, and spoke fluent Russian. But he didn't trust the Soviets, and they didn't trust him. He launched the Korean War in 1950 after the Soviets told him not to. They were trying to consolidate their gains in Europe, and rebuild from WW2. The last thing they wanted was a bloody and expensive war in Asia. During the Korean War, China helped NK way more than the Soviets did. After the war, rather than showing gratitude to his allies, Kim Il Sung purged the NK government of anyone suspected of leaning toward either Russia or China. Korea is not called the "Hermit Kingdom" for nothing.