That sounds really odd. My other suggestion would have been that your research was too high for your population, but with only 12 research facilities, that's probably not the case. My main suggestion would be to get going and start colonizing. Multiple colonies are really necessary for a healthy economy in my experience. The reason for the ping-ponging might be due to different sub-pulses (same as with mineral stockpiles and mass drivers).
Again, it might be helpful to see your wealth/trade overview for the past year. If useful, I have attached my own for comparison. 18 years into the game, starting population of 750m, current total population of 1211m with three populated colonies.
So I started a new game to test out some things about the economy. Conventional start; Sol; baseline human.
- First game I placed 1500 CI and 350 pop; about 12 mil worker shortage +23 to budget first two years whilst researching TN 6 labs
1a. TN complete and population has grown to 400 or so; worker shortage is 3. 5 mil; 15 finance centers +6 to budget 8 labs
1b. Start conversion of CI to TN; 200 wealth research complete; 2 labs in que; 65 finance centers +0 to budget; 3 mil worker surplus
Photos of year 3, 4, and 5 yearly budgets. No exploration and colonization done so far. I am going to run till year 10 and first colony on luna to see if there is an increase, but I'm feeling that you have to start with 500 mil+ pop to see a positive in finance and also build the finance centers to make them a sizable amount of your total worker population to run positive.
That will be my next run through to see how normal population goes. But at this current stage in playing it seems that unless you don't expand within the first 5 years, you get a bust and boom cycle that has to be balanced getting those essential techs and building more finance centers. Thoughts?