Hi,
I need some help here.
Driving with cruise set at 74mph. Hit the gas and the engine begins misfiring around 80mph. Let off the gas and cruise control speed goes to 78. Come to a grade that usually make no difference and the car slows to 72 to make it. Get over the top and speed goes up to 76 before settling back to 74. Normally my speed stays within .5mph of the set speed.
engine breaking up at high speed. Doesn't seem to matter the gear or rpm.
I changed my plugs and even my coils.
Any thoughts?

 

If you still have a catalytic converter, check it to see if it's clogged.
Misfires kill cats.
Second thought would be possibly the fuel pump dying.

 

Thank you.
I have no CEL or codes.
Live data at 2000rpm's after warming up reads:
O2B12S12 (V) float between .5 and .7 but will occasionally dip to .2
O2S11 (mA) float between -0.043 and 0.023
Cat temp 1489.24 F
From what I have read this means my Cat it OK.
Still trying to look up how to check my fuel pump.

 

Generating Power at Full Throttle
• Ignition Coils
• Spark Plugs
• Spark Plug Wires
• Engine Compression
• E-Shaft Sensor (ESS)
• Air Filter
• Intake Valving
• Throttle Body
• Mass Air Flow Sensor (MAF)
• Fuel Pump
• Fuel Injectors
• Catalytic Converter (Cat)

Keeping the Engine at a Cruise
• Ignition Coils
• Spark Plugs
• Spark Plug Wires
• Engine Compression
• E-Shaft Sensor (ESS)
• Intake Valving
• Throttle Body
• Mass Air Flow Sensor (MAF)
• Fuel Pump
• Fuel Injectors
• Front O2 sensor
• Catalytic Converter (Cat)