2005 ext trailblazer has a problem with the oil pressure gauge. the check presure light on the dash comes on that the pressure goes back to normal. get a ticking noise from engine when this happens?

Have you checked the pressure with a mechanical gauge to make sure the gauge on the dash is correct ?
And did you check the rod and main bearings for excessive wear when you replaced the pump ?

Gauge problem ,bad stepper motor !
How to repair GM Instrument Cluster Speedometer Gauge Stepper

The oil sensor sending unit is shot,

At idle, the gauge usually reads about 40, when engine revs up, the gauge will go up to about 50 or 60. This totally normal and you should know that the reading on the gauge is just made up by the computer because the engine does not have an actual oil pressure sensor.

You may have a oil pressure switch going bad or your idle is too low. The idle should be around 600 RPM. The oil pressure displayed on the instrument panel gauge is not an actual reading, it is made up by the computer. You probably should clean the throttle body as well, make sure to clean both front side and rear side thoroughly. Also disconnect the battery while doing the cleaning. This will allow the computer to reset and relearn.

I can't speak for the engine light issue, but look into stepper motors for your gauges for the oil pressure issue. I just purchased a used '04 Trailblazer EXT and the tachometer stopped working 1 week after driving it off the lot. I did some looking and apparently the Delphi motors that GM used do not hold up. "Check this link for more info...
http://forums.trailvoy.com/showthread.php?t=48886

could be the oil pump in the motor.

You could try thicker oil or stop using synthetic or add some lucas oil treatment or change the oil. The sender could be bad too. A diag is in order because if there is no presure the engine will soon die.

I have this same problem with my 2002 trailblazer; however, I goofed up and had a mechanic look at it, since I was perplexed as to what was going on with it. He told me the oil pump needed replacing and it ended up costing me around a $950. Needless to say, it still happens. A grand down the drain - it wasn't the oil pump. The oil level is always good. This generally occurs when I've run the vehicle for a while and then come to a stop. It occurs quite often during the spring/summer and doesn't happen all that much during colder months.

Sounds like the sensor is failing.Have oil pressure checked with a mechanical gage to confirm. A new sensor or wiring repair is allot cheaper then a new engine!!