When an engine that was running well begins to act up, replace the glow plug. That is ALWAYS the first thing I check. 99% of the time, that will fix the problems.

If that doesn't fix it, pull the carb off, remove the needle valve and the barrel, and make sure all the small passages are clear. A small particle can be easily trapped in the needle or spray bar assembly. After reassembly, go through the tuning process again.

If that doesn't fix it, check all your fuel lines, including INSIDE the tank for any small holes. Oh what the heck, fuel tubing is cheap, and since you've got it out of the plane anyway, just replace the plumbing.

If that doesn't fix it, try new fuel.

These four steps should fix 99.99% of the problems. If these don't fix it, then you need to start looking for other airleaks that might have developed. Are the head and backplate screws tight? How do the bearings feel? You may need to remove the backplate to visually inspect the rear bearing. Is the piston binding? Is there still good compression? How old is your fuel?

Good Luck
Brad