I did not have time for a longer explanation last night, so I thought I would elaborate on my first response.
If your car runs okay at lower speeds and accelerations, and craps (technical term) out when you apply higher load levels, more than likely it is starving for one or more of the essentials; air, spark or fuel.
Air: check your air filter. If it is dirty, change it.
Fuel: when is the last time you changed your fuel filter? If it has been a while, and even if it hasn't, change it. Its a $10 part and half hour of your time.
Spark: deteriorated wires and marginal spark plugs may operate fine under normal loads, and not be up to the task under high load. You can check your wires using the procedure from the thread I gave you yesterday, but if they haven't been changed in a while you should get new plug wires. Same for spark plugs. I could point you to numerous threads that tell you how to inspect them but I recommend (based on your performance issues) you should just change them. Next, follow the information I provided in the previous response to check your ignition coils.
If you change your air filter, fuel filter, spark plugs and plug wires and are still having the issue, the next thing you will want to do is get your car checked for DTC'* (diagnostic trouble codes). There is probably a parts store near you (Autozone) that will check this for you at no charge. You also can invest in one (Ebay $30 to $40). Your car is OBDII compliant. What we are looking for here are misfire codes to tell us if you have random misfires, or misfires on one (or two) specific cyclinders.
For example, if your misfire is coming from cylinder #5, you can switch the coil pack from cylinder #5 with one of the other coil packs. If the problem moves with the coil pack, the coil pack is bad. If the problem does not move, the next step is to switch the fuel injector from the misfiring cylinder with another one. There are also checks you can do on your injectors and injector circuits, but we will get into that later.
When I experienced the same thing you are, it was a cracked ceramic housing on one of my spark plugs. Under normal loads it would fire fine, but under high loads it would arc out to the engine block and misfire.
Keep us in the loop on your progress, and we will walk you through this.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)