A/C High & low gage pressures low & suction line is 10 degrees cooler at the condenser than at the evaporator?

Boss replaced condenser with 13 SEER 3.5 ton, too much charge was added to compensate for longer line-set. Had a "Z" evaporator and TXV, airflow good. When system started low side was 75 PSI and high side was 198 PSI. Outside 84 F. Then low side would slowly drop while high side remained at 198 PSI. Eventually dropping enough to frost suction line at condenser then freeze up evaporator. Low side got down to 36 PSI at one point before I shut it off and defrosted it. No temperature drop on liquid line or across L.L. filter. When I last checked system, low side around 40-45 PSI high side 196-198 PSI, suction line at evaporator was in low 40s F. and at condenser was low 30s F. Subcool on L.L. was 0 possibly even a very little superheated. Superheat on S.L. at condenser was high at one point (earlier) then I believe was dropping probably around 10 when last on system, didn't really concentrate on it since it had TXV. My original thoughts were bad TXV then suction line restriction since there was temperature drop from evap. to condenser. Now I'm thinking low efficiency condenser with high efficiency evaporator. My boss decide to replace line-set and evaporator with one matched to condenser and now system is fine, but I would still like to know/understand what was going on.