My Cruiser begins to overheat after 20 miles of me driving at least 50 MPH. Turning the heater on brings the temp down.

You might have air lock in heater matrix when bleeding it put heat on full an nd let it get to temperature and bleed it then ..I had a Volvo f12 globetrotter that would overheat coming up Mont blanc at minus 20 outside temperature it was radiater changed perfect after

150 miles at least also check ur air filter change nice clean one

You mean PCM most states read the readiness monitors depending on the state on how many they allow not to be completed. The first one on the list that is hard to get run and pass is the EVAP Because this requires the car to be cold and fuel tank must between 15%-85% and both air temp and coolant temp with in 10 degrees of each other. Start car let idle for 5 -10 minutes Then drive car normally no hard accelerations or quick stops bring speed to 50-60 mph and coast down to 20-15 mph without brake pedal input repeat this twice.Then bring speed to 58-62 mph and set cruise control run for 12-16 miles exit freeway caught a red light or stop and idle 2 minutes shut car off rest for 5 minutes and repeat freeway drive. then redo morning cold start again . then have some one test the generic readiness monitors to what needs to run and attempt again. California before 2000 1 can be incomplete after 2000 all have to be completed which is hard some will not run for 800 miles.

what motor do you have? check all the hoses see to see if any are leaking and see if the coolant level is full. may have a stuck thermostat

Maybe your radiator is clogged.

It sounds like your heater core or hoses that feed it has blown.

To review - - after you disconnect the battery you need to drive the vehicle at highway speed for 20 - 50 miles to allow the computer to "re-learn" the data parameters. This means getting the engine up to operating temp and speeds of at least 50 mph.
If the engine does not reach operating temp some of the sensors will not be active yet.

U have to operate the engine at all kinds of diff speeds and drive modes, it is called a OBDII drive cycle, here is how u do one.
1. As soon as the engine starts, idle the engine in drive for two and a half minutes with the A/C and rear defrost on. OBDII checks oxygen sensor heater circuits, air pump and EVAP purge.

2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle. OBDII checks for ignition misfire, fuel trim and canister purge.

3. Hold at a steady state speed of 55 mph for three minutes.

OBDII monitors EGR, air pump, O2 sensors and canister purge.

4. Decelerate (coast down) to 20 mph without braking or depressing the clutch. OBDII checks EGR and purge functions.

5. Accelerate back to 55 to 60 mph at half throttle. OBDII checks misfire, fuel trim and purge again.

6. Hold at a steady speed of 55 to 60 mph for five minutes.

OBDII monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and purge functions.

7. Decelerate (coast down) to a stop without braking. OBDII makes a final check of EGR and canister purge

Well it turns out that I didnt get all the air out of the system. so I top off the radiator, and had it pressure tested. All is working well. Thanks anyway.

Allow it to cool down and refill system while running with heater on hot. Hopefully this will get the air out of the system. However, if this condition persists, have the radiator checked for hydrocarbons. These engines are sensitive to overheat and you may have blown a head gasket. (hope not)
Good luck!