Welcome to the forum

There are a number of things that could be causing your problem. Here are some questions:

Did you do a search of this forum for "STX38 PTO"? (This is a recurring problem, and has been addressed a number of times.)
Does your STX have a black mower deck, or is it yellow? (Yes, this matters, because other differences include the PTO switch.)
Do you have a multimeter, and know how to use it?
Does your battery have a full charge (even with the engine running, a weak battery can fail to provide enough voltage to activate the PTO.) (My first guess would be that a weak battery is your problem.)

However, your statement that the PTO switch has been "touchy" could well indicate a loose or corroded connection. The actual switch itself usually either works or doesn't work at all.

First, sit in the tractor seat, depress the clutch/brake pedal, turn the key switch to the "on" position, but do not start the engine. Then turn the PTO switch "on." You should hear a "click" from the area of the PTO, but I expect that you will not.
The second - and easiest - thing to check is the ground wire that is attached to the underside of the tractor frame, near the right front wheel.
Third, unplug the electrical connector at the PTO and, using a multimeter, check for battery voltage at that point (the seat must be occupied, key switch on, and PTO switch on.) If you're getting NO voltage, the problem could be a malfunction of the key switch, the seat switch, or the PTO switch, a short circuit, or a bad ground. If you're getting less than 12 volts, you've probably got a weak battery or a corroded connector or ground.

If you do these things, report back with results, and we'll have a further shot at correcting your problem

You need to go the the LAWN TRACTOR FORUM on Gardenweb, and search on STX38 PTO switch. This forum isn't dedicated to lawn tractors.

Here's a link to something I posted on the LT forum several years ago. The switch testing steps are for a black deck STX

Here is a link that might be useful: STX38 PTO switch testing

You would be better off to find a friend that has a knowledge of electricity to help you rather than to keep buying parts for it..it would be a lot cheaper, even if you had to buy his dinner.