1994 Cadillac STS hard shifting

If you have a check engine light, that may explain the hard shifting. Depending on what code you have, the system could take action to command max line pressure, and the results would be firm engagement, and firm shifts. Slipping when cold could be the result of leaking piston seals inside the transmission, that soften and seal as the fluid temperature increases. It is possible, the cause of the cold slip, could be also the cause of the defaulted max line pressure.