another yellow leaf question...sorry...

Okay. I know that there's probably been a million posts about this, but despite a good search and a look at the Overgrow FAQs I have found nothing.

Here's the prob:

The leaves on the bottom of the plant are yellowing and then falling off. It only happens to one set at a time (two opposite leaves) and as soon as they are terminal, the set above starts to die. At this rate I'll have no plant left in a week or two.

In my last grow I experienced a similar problem, and was told by a friend it was lack of ferts...then water....
I have to disagree with him because they were perfectly watered (soak every three days and wait until nearly dry) with no dry patches inside the pot at all. I also upped the dose of the fertilizer but this seemed to make things worse. So then I tried it the other way, lower fert, and they continued with dropping the bottom leaves.

I know that this is natural to some degree, but the rate of death for these leaves is quite scary. Maybe it is strain-related, as the seeds are the same as the last batch (just bagseed - but good).

Can anyone give me a clear, concise explanation on what is happening here.

Oh, by the way, I'm using Miracle-Gro *hangs head in shame*