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It sounds like your vehicle might be in "limp home" mode. This is the mode many cars go into if the car's computer senses a major engine/transmission problem. It will limit the car's speed and RPMs in order to preserve any engine function that may remain, with the intent of "limping" you home.
My first thought is a plugged exhaust, but also there could be a problem with the MAF (Mass Air Flow) sensor. If you are technically inclined and have a vacuum guage, you could check the amount of vacuum at idle. It should be over 20 inch lbs. If not, and it declines the longer the engine is run it's the exhaust.
You could also be having a starving fuel system while under a load. Bad fuel pump, clogged filter, clogged injectors, low voltage at the fuel pump or a bad pump itself. Get a manual at your local parts store for reference.
Your brakes could be dragging. Jack the car up and turn the wheels to see if they turn freely. It's a long shot, but could be the problem. Is the parking brake off? As obvious as this problem is, it might be wise to take it to a technician and get a diagnosis. Someone versed in repair will be able to detect what is wrong.
It's unlikely it's the canister purge solenoid/valve.