1995 GMC Suburban
Has to be fast idle and up to temp so the t-stat is open.Have you tried just parking it and putting it in fast idle to see if it is leaking externally? (such as hoses, pin hole in the rad, overflow tank) OK, dumb question, but had to ask...
It's either an external leak, or an internal one. If external, you should be able to locate visually. If not visible, then it must be an internal leak. Since you say that the oil level is OK, then it must be leaking into the cylinders and leaving via the exhaust.
JUst my $0.02...
Some weak spot, the coolant cross over usually have slow leak. Downstream from that is the "T" which is made of plastic, which can get brittle overtime. Replace it with brass or metal T.
All other hoses.