last couple times I have taken the boat out, it seems to fish tail a little bit while on pad. scares me. Anyone know what may be causing this?
What boat & motor is it? I would bet it is chine-walking. The guys above are correct when they say your boat will run faster in the cooler air and colder water. When the hull gets "freed up" from the water, you are basically balancing the boat on the pad. The torque from the propeller is trying to knock you off the balancing point.
You definitely want to make sure you have no slack in the steering system and that your motor mounts are intact. Even the newer hydraulic steering can develop slop if you have air bubbles in the system. With the boat on the trailer, turn the wheel slightly from side to side. The motor should respond to the movements. If it does not, you have some air in the system or you have some loose bolts in the steering column. This can lead to Disaster if left unchecked.
Watch this to see what can happen if the steering fails. The boat can do a 180 degree turn at high speed and eject you & your passengers. It was discovered that the lock nut on the steering arm from the steering system to the motor tiller arm had vibrated loose and came off causing this accident. These two young men were definitely very fortunate. The Good Lord had His hand over them that day. https://www.youtube.com/watch?v=1AkgjPkCQXc
The hull will begin to oscillate side to side in what is called chine-walking and is common with lighter high performance hulls like Bullets & Allisons. Older Tritons are also known to do this as will some BassCats. This is completely normal at high trim angles. You can trim down to lessen the effect as one guy suggested.
The trade-off is, more wetted hull surface, more drag, less top speed. Very small corrections can be made to the steering wheel to offset this oscillation but it takes lots of seat time & practice. The best thing to do is simply trim the motor down a bit to minimize the chine walk.