Power Drain

I have a 2001 Toyota Camry with 175,500 mi. I have noticed a power drain at first mostly when going uphill though now on straight aways. Usually this occurs at speeds of 55 mph or greater. The car begins to jerk and loses speed rapidly. I regain power when I left off the gas pedal. Also, when I turn the A/C on at any speed, there is a loud clunk which seems to come from the rear of car, causing it jerk again and draining even more power. A/C does not blow cool air on any setting except high. I checked all the fluids: transmission, power steering, oil. They are full. Any suggestions? Ideas?

The lack of power could be from a dirty fuel filter. When was the last time that was changed?

The fan speed problem could be with the blower motor resistor pack or with high speed blower motor relay.