1994 olds lss heater

my wife bought this car new “her baby” in august we had the upper intake manifold gasket replaced, only real major thing done to it. when it got cold and needed the hearter she got hot air on drivers side and COLD air on passengers side. i disconnected the linkage for the ent to that side so it would only get heat. worked for that, then got cold air on drivers side (?). i rehooked the vent and replaced the ambiant sensor behind the grill. there is hot air on passenger side, warm air on drivers side…not like it used to be. can you tell me what i might look at next??? the air filter was replaced right after the intake manifold gasket was done. temp. of engine is good, there is coolant, moving switch on passengers door arm rest does move damper for vent ok, the digital read out on dash for the outside temp and setting heater temp flashes (goes on and off) when car is first started then is ok…this was just an idea for changing the sensor. sure could use some help…thanks!!!