I have a 1999 gmc yukon and when i drive down the road at about 15 - 30 mph the rpm's drop and the tranny acts like it wants toshift up and then back down again and then up again about three times.i had the top part of the engine apart replacing the lifters..and fixed that but now after lifters are reaired i have this problem.What would coase this and how can i fix it??

Common problems
1. fuel filter clog and fuel pump stop working
2. Crankshaft sensor (spark is cut during high demand)
3. Air filter clog (not enough air during high rpm
hope it helps

change oil- if high milage step up one grade duirng warmer weather

it sounds like the front tires need balancing.if you drive the truck slowly(creeping down the road at about 5mph)and lightly holding the steering wheel do you feel the steering wheel moving back and forth.this could indicate a bad belt in a tire where as a wheel imbalance won`t show up at this low of a speed,but a bad belt will.also have the front end checked for worn front end components and wheel bearings,this can cause a bad shake too.

I would suspect tire balance or a bent rim. Have a shop that has a road force balancer check the tires/wheels.

problem throttle body or accelerator position sensor
there will be codes but it will need somebody experienced with the right scanner protocol to read them

What you have is the torque converter clutch,cycling on and off.
When it is off.your rpm will go up and your milage will go down
First-check to see if your brake switch, on the back of the brake peddle, is loose or misadjusted. When you apply the brakes,the first thing to happen, is that you unlock the converter,so your trans is not connected directly to the motor,or you would stall. Then below 42 MPH,your trans is working off the converter, as a pressure multiplier ,based on rpm
Second- switch okey. You remove trans pan and change TCC Solenoid.

Let me know if if you need more guidance

More then likely you have a faulty throttle position sensor (TPS)

well your tranny is slipping check your tranny fluid make sure its not low on fluid

check your torque converter and solenoid to ensure it doesnt need replacement hope this help good luck

The voltage regulator is having a problem. It is built into the alternator. You should take it to a local parts store that does free testing, and have it tested out.