Oldsmobile/Buick Repair: valve chatter and tach meter not workg at same time, gas milage, gm dealer

Question
2001 lasabre 88000 mile ,  intermitten ly the loss of tach and valve chatter seems to happen at same time .  the car can be turned off and restarted it is normal again ,  it is getting more frequent.  this happened a couple to three years ago once then in the last year it has incre/.  gm dealer has replac rear oxy sensor and cable to it due to corris.  and guess it may cause prob.  it has reocured twice in five miles.  gas milage is still about 25 avg city.  3800 eng.  i have added gas treatmnt to tank to see if it would help.  

Answer
Hi M,
I think that would be spark knock, rather than valves.
First thing to do is wiggle all connections, but if you have corrosion in one connection, I might unplug several of them, and see if there is more that needs cleaned up.
Clean tight connections are a must. Not just battery connections, but every connection.
It sounds like an intermittent loss of connection somewhere.

Van