5000cs Radiator Leak??

[S P SHERMAN]

My 87 5KTQ has had a slow coolant leak for a while. Idiot light comes on typically after a week or so of driving. I thought I had found it, when I spotted a leak under pressure at the tiny overflow tube between the radiator and overflow tank and replaced that tube.

Unfortunately still have s slow drip from under the radiator when the coolant is hot and under pressure. I took a look right after driving, and saw wet spots in the middle of the radiator and at the bottom of the radiator, both rear side in the core area of the radiator. Have to admit, I was surprised, as this is a fairly new rad, about 5 years old, put in by me when the old one's nipple broke off.

So my questions are:

1. Any other possibilities here other than a  leaky core on the rad?

2. Is it even worth trying any of the stop leak products. I'd really like to be able to use this car as a snow car for another year, and if there are any brands that work better. Recommendations?

TIA