We installed a bath sink that repaced similar one... older house, sink drains into floor not wall... has a "s" pipe, as before, but now won't drain after water runs a minute... then it gulps and a bubble comes out. Then drains very very slowly. Help!?

I don't know what you could have done but it sounds like your vent is not clear. The gulps is air bubbling back. Remove your S trap and try some rodding about 4-6 feet with an auger. S traps are now illegal but you get to keep yours, grandfathered.

we installed a bath sink that repaced similar one....older house, sink drains into floor not wall..........has a "s" pipe, as before, but now won't drain after water runs a minute....then it gulps and a bubble comes out. then drains very very slowly. help!!??

Does the new sink have an overflow?

If you've installed a "S" trap then it's venting through the trap,(your gulp and bubble). Try installing a AAV,( Air Admittance Valve) just after the trap. Cut in a tee in the vertical drain coming out of the "S" trap and install a streel ell facing up. You my now instal a AAV on the street ell. This will vent the lavatory and relieve the suction that's stopping the flow. Good luck. Tom