Stumble and loss of power

My 1999 Olds Alero loses its power steering for short periods of time. The problem seems to happen more often when I am turning sharply at low speeds. The fluid is slightly low but I haven't noticed any leaks.

After driving the car for a while the car would not accelerate past 30mph. I was told the engine is not any good, but I would like to make sure before I junk the car. I purchased the car at an auction and I drove the car home without any problems from Elkridge, Md to Balt. City. Later the car starting cutting off, and I had to get it towed. The mechanic put a new Coil Pack and Modulater and new brakes on the car and I purchased new spark plugs but they weren't put on because another mechanic said the car wasn't any good.

Electrical problem
1999 Oldsmobile Alero 6 cyl Two Wheel Drive Automatic 100K miles

So for about a month, when I drive my 1999 Oldsmobile Alero if I go over 60mph the speedometer will start dropping and bouncing. If it continues for 10 seconds, the RPMs will go up by about 500. Eventually it will fix itself, but sometimes my engine will kind of stutter with a minor loss of power. My mechanic thought it was my solenoid pack (?) And he replaced it, however my car is still doing the speedometer bouncing and RPM things. What could be causing that and how do I fix it?

No the odometer continues working just fine, as far as the transmission shifting it shifts fine all the way. At least I think so, I've never really noticed that it's shifting or trying to shift gears. The speedometer bouncing only occurs after driving at 65 mph or higher for a good length of time. The amount of time varies, sometimes only 10 minutes and sometimes it's over 30 minutes.