'99 silverado 4x4. code p0306 is what comes up on code reader. truck feels like its not running on all cylinders. loss of power.just had new water pump put on and dual exhaust. any ideas?

Here's a link about your code P0306. P0306 OBD II Trouble Code

Have it checked with a code reader. You have a dead cylinder. The code reader will tell you which cylinder it id. That cylinder needs a fuel injector change. Symptom being, when start moving and normal throttle, shifts thru the gears fine with no loss of power. As soon as it goes into Overdrive, it feels like the whole car is going to vibrate off its chassis. That thumping is what we call a dead cylinder. Because it is just that, dead. Yet when you push the pedal down, it will downshift and run fine. Change the injector.

Well it really doesn't matter much. I would probably clear the codes and see which ones come back first.
Code 123 is for the throttle position sensor signal being out of range. Could be the sensor or a wiring problem.
Code 155 is for a problem with the heater circuit going to O2 sensor bank 2 sensor 1. Thats on the side with cylinder two if you have a V6 engine. Could be the sensor but its usually the lack of power for the heater.
Code 306 is for loss of power from cylinder 6 which could be mechanical, fuel, or spark related.
The three codes are not connected to each other. And trouble codes are not a statement to replace parts, only what to test.

all time 4wd or not??

It overheated because the cooling system hasn't been bled properly. When that happened it was detected by the ECM who cut off power permanently to prevent further damage.
You'll have to delete the warning from the ECM with a code reader for the power to return.

Random misfire. Computer cannot figure out which cylinder is misfiring or it occurs randomly on all 8 cylinders. A friend of mine found his ABS system was kicking in while driving down the road, feeling like the engine was missing. Computer detected it as a P0300 random misfire. He removed ABS fuse and eliminated the problem.

Cauases or fault codes are not cut and dry, but here are the possible causes of codes P0316 and P0306 which are related codes.
P0316 - Engine Misfire Detected on Startup
Symptoms - Engine Light ON (or Service Engine Soon Warning Light)
- Engine fails to start
- Engine runs rough Possible causes - Crankshaft Position (CKP) sensor harness is open or shorted
- Crankshaft Position (CKP) sensor circuit poor electrical connection
- Faulty Crankshaft Position (CKP)
- Ignition System
- Fuel Injectors
- Running out of fuel
- Fuel Quality
- Faulty Engine Control Module (ECM) P0306 - Cylinder 6 Misfire Detected Symptoms - Engine Light ON (or Service Engine Soon Warning Light)
- Lack/loss of power
- Hard start
- Engine hesitation
Possible causes - Faulty spark plug 6
- Faulty ignition coil 6
- Ignition coil 6 harness is open or shorted
- Ignition coil 6 poor electrical connection
- Clogged or faulty fuel injector 6
- Injector 6 harness is open or shorted
- Injector 6 poor electrical connection
- Improper spark plug 6
- Insufficient cylinder 6 compression
- Incorrect fuel pressure
- Intake air leak

Misfiring on #6 cylinder. A code P0306 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer

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look at the plug wires they are known to bust when old near the boot

Perhaps the compression guage u used has a defective schrader valve, if the compression was that low it would run bad, and mileage would not be 19 MPG. Glad to hear all is well.