This reminds me of a 1978 Dodge Ramcharger I bought. The rear side windows were a little too small for the openings. The first heavy rain water was coming in like crazy. The service manager at the dealer tried to tell me all cars leak. I said that's funny because my 1977 Honda Civic doesn't leak at all. Any way they took some black sealant between the glass and the gasket and used it to force the gasket back towards the window opening. It came out looking good and stopped any further leaking. Sounds like your window is shifting in the gasket. If it can be re positioned maybe sealant will help keep it in place.