The loss of capacity when changing to 134 is significant. These cars do not sufficient excess capacity to tolerate this loss of capacity.
Possibly the system is just low on refrigerant causing the degraded performance. If the refrigerant is not low, however, you have two choices; you can add a parallel flow condensor that will help some, or you can completely flush the system, change the filter drier, put in mineral oil and go back to R12.
The conversion from R12 is false economy in most cases. You are only working on the system when there is a problem of some sort. The other expenses of repairing the system always exceed the cost of the R12.
Now that you have converted, you are faced with even more expenses. This is a good example of why not to convert to R134.