My 2-cents worth is that if you've had this car for nearly 33 years, you're obviously taking care of it, including changing the fuel filter on a regular basis. True, gas at the station can be contaminated, but aside from moisture contamination the filter should take care of that. Rather than go through all the trouble of cleaning the fuel tanks, consider that what you're experiencing might be due to fuel pumps that need replacing. The E21 has two pumps, a main one and a kind of "pre-pump" located in the fuel tank itself. My guess, and it's just a guess, is that the pre-pump is worn, and needs to be replaced. I don't know what the diagnostics are for checking it out, but I suggest doing that first before more dramatic measures.