Two problems-engine related

Bought my daughter her first car the other day. Its a 94 Buick Century
with the 3.1 V6. It has 119,000 miles on it.

First problem: The lifters clatter excessively. I put a quart of Marvel Mystery oil in it to see if that would loosen up some possible sticky lifters and we have ran a tankful of gas through it but lifters are still just as loud. I know they are not adjustable so what next?

Second problem: The car just doesn't run right at normal driving. It seems to surge a little while gently accelerating. Also, doesn't seem to have proper amount of power when taking off from a stop unless you give it more throttle than is normally necessary. Pretty sluggish.
On the other hand, if you nail it hard, it seems to pull strong and doesn'
t miss a beat.

I have pulled the plugs and they all look good. I did replace plug wires with a new set and ran a bottle of fuel injector cleaner through first tank of gas.

I have not checked or changed the fuel filter although I have doubts this is the problem since it runs good when you hammer it so its getting enough fuel at full throttle.

Could the problem be the oxygen sensor? Thats what my friend thinks it is but a new one is about $54.00 and I hate to start throwing money at it just guessing it will fix it. There is no check engine light on.

My friend has a scanner and the only code it spit out was a PO705, transmission range sensor circuit malfunction. What the heck is that?
Transmission works fine!

Anyone have any ideas? Thanks in advance for your help.

I have a 94 Century and when I started it in the morning the lifters would chatter until I revved it up, or until it warmed up. Mine has 149,000 miles on it as well.