95 fleetwood caddy 5.7L- dies while driving and cooling problem(s)

I have a 95 fleetwood brougham 5.7 v8 rwd...when i drive for a period of time or just idle the engine in traffic I see light steam coming out of the defrost and a/c vents with no a/c or defrost on. I've only had this car for 1 year and last summer I changed the t-stat and gasket, hopefully my water pump is ok., it seems to flow but it also seems that there is blockage somewhere. I was also told that my heater core is leaking..how can I check to see for sure?? I've also read some of the posts where you guys recommend cleaning the fins for debris. I've had a oil pump,pcv valve and fuse and filter all changed and a month later it was mis firing tremendously., now I have changed spark plugs, wires, cap and rotor kit, ignition coil and my car is still missing when I drive for 10 to 20 min and it seems to want to overheat but it doesnt, I just pull over and wait then it starts again but i cant get up to 45mph before it kills again. I got the third diagnostic:banghead: ran on it and this time I was there to see what codes came up, and they were - (16) "low resolution pulse circuit" and (43) "knock sens circuit"
now, I changed the knock sensor myself but that other code, could that be the distributor or pick up coil?? does my car have a pick up coil(magnetic)??
PEASE HELP ME I'M RUNNING OUT OF TIME AND I BEEN RAN OUT OF MONEY!!!--ANY SUGGESTIONS OR ADVICE PLEEEEAAASSSSEE??!!:frown::frown::frown:

Welcome to AF. More than likely your heater core is leaking, doing a pressure test and look under the heater box will confirm this. When it dies do you have spark and fuel presssure at the rail, this is just basic diagnostics.