Ok, this tax revenue bug, intentional or not, was hard for me to figure out how it was happening.
When the tax revenue bug or "feature" occurs, your tax revenue drops by a percentage equal to around half your current tax rate. (i.e. 1000 bc tax revenue will drop to 800 bc with bug if your tax rate is 40%, since 20% of 1000 is 200)
This only affects tax revenue and not overall income.
As for the cause, I FINALLY pin-pointed it. This bug or feature occurs whenever a race, human or A.I., gets over 20000 bc.
This definately explains why my economic graph fluctuates every turn or so once I get around 20000bc, spend some to get below 20000bc, then go above 20000bc again when tax revenue returns to normal, and have it drop once again as soon as I go over 20000bc.
This has worked for me on every difficulty level I've tried. Normal -> Crippling
Was this intentional?