What mite cause it to chug some when it hits about 45 miles per hour when increasing speed?

check the vacuum hose on the fuel regulator this increases the fuel pressure under load

releasing the gas pedal and it gets going again
not very safe

is the engine light on
could be any faulty sensor in this engine diagram

maf clean might help
if a spark issue needs chasing
it could be a fuel problem
maybe bad fuel or blocked fuel filter

When you replaced the spark plugs did you make sure they were properly gapped to your make/year/engine. Have bought many "pre-gapped" sets of plugs, only to find that they aren't as advertised. Also, your catalytic converter may be becoming clogged. Relative temp and humidity can make this an intermittent problem. I have purchased Chiltons/Haynes repair manuals for all vehicles I have bought. The information, especially under trouble shooting is invaluable. I admit, when I got rid of a vehicle, I kept the books.

Sounds like you have a vacuum leak. Although, it could be bad o2 sensors. If the computer sees that the o2 sensors are showing lean then it will increase fuel flow to try to even out, which causes it to burn more gas.

If you have done basic tune up stuff, I would check fuel pressure with gauge installed and running down the highway. Other possibles are bad ignition module, loose wire somewhere in coil circuit, bad fuel pump, filter, many others.

could be a plugged fuel filter

Hi.

Problem is usually the speed sender failing when speed gets too high. In some case the speed gauge itself is damaged. In Mercury the speed sender is located in the transmission. The part is difficult to test without professional equipment. Repair is done replacing the speed sender. If that does not fix the problem, work on the instrument panel speedo.

Here location of the speed sender:

Try replacing the throttle position sensor. They can get 'noisy' electrically which confuses the computer, yet doesn't set a code for the defect. This sensor is supposed to deliver a smooth transition from near zero DC volts to near +5 volts as the throttle position is increased. Hope this helps!

Yeah you may need a whole new dashboard because the whole panel circuit for may be messed up.

Power drive delays the shift points for upshift of the automatic transmission. It will degrade fuel economy, but you'll get more peppy performance.

Winter drive will only engage if the vehicle speed is below a certain threshold (I think 23 miles per hour) - and only if the vehicle is in Reverse, Neutral, or Drive. It causes the automatic transmission to start off in 3rd gear (rather than 1st gear). The purpose is to provide power, but not torque to the tires. In winter driving, torque increases the probablility of the tires spinning. The vehicle will seem to be VERY sluggish when winter driving is engaged - which is completely normal behaviour. It will automatically disengage when vehicle speed increases above the threshold.