I have an '07 CTS base model with 83000 miles at around 1700-2000 rpm with a load on the engine it kind of shutters/bucks,when i give it more gas it downshifts and is fine. what causes this?

Sounds like an ignition misfire. Spark plugs or coils/boots. I'd start by putting new spark plugs in (cheapest place to start).

If its a misfire the check engine light will flash does your check engine light flash when this happens. Or at the very least turn on?

No, it will not always flash for a misfire. The misfire must must happen for a good duration for the PCM to set a code. It's an internal combustion engine, it will misfire from time to time. There are no exacts here. The computer won't set a code for a couple of hiccups. If it did, you would be sitting at a dealership/repair shop, every day. There can be a misfire code, in conjunction with other misfire codes (cylinder 1 "2000 misfires", cylinder 6 "500 misfires"...... Although the engine computer is fast, it sometimes can't catch the exact cylinder that is misfiring, if at all. It has to be consistent, for a set period of time. A scanner with live data, is the best way to be sure which cylinder it's on. The cylinder with the highest number of misfires is usually the culprit. This can be done by a local educated technician. I stress the word "technician". There are mechanics, they can fix your car and your pocket book, and then there are technicians that might stress the pocket book, but get the job done right.

With his caddy the crank shaft position sensor and camshaft position sensor allows the computer to know where the engine is in its cycle at any given moment. Those sensors also detect every single misfire little or big and adjust timing fuel and spark duration to that. If the misfire that was detected is bad enough, the check engine light will trip as soon as it happens. If the same misfire is detected 2 more times in the next 4 cycles that's 3 out of 5 cycles which is 10 engine revolutions then it is considered bad enough to make the check engine light flash. So technically the light could come on after 3 cycles or 6 revolutions. So at 720 rpm that's 12 revolutions per second which is 6 cycles. So your check engine can come on and start flashing in a matter of a half a second. At 1440 rpm the engine could be fine and in 1/4 of a second the light can come on and start flashing due to a misfire. If the computer was confused as to where the engine was and didn't know what cylinder needed to fire then how would the engine run?

I seen on you tube that if your engine does not have the correct amount of oil in the engine it can also make the engine misfire. I am having the same issue with mine. The check engine light started to flash right after having my oil changed. I did check the oil and found it to be one quart over. Not sure if oil could play a part but I'll find out today.

I’m having the same issue with my 09 cts 3.6 while driving at about 2000 rpms the gauge goes up and down a little bit. The car accelerates fine. It’s inly while I’m maintaining the same speed and my rpms are at 2000