I have related this before in perhaps more detail, but the short version is that the symptoms you cite also match a known issue (at least with the Brodak .25):
If the case and/or muffler interface has not been machined or lapped it will distort the piston/liner fit. It may feel nice when cold, but once run, it starts to bind up. Check your case and muffler. We lapped the muffler on my friend's engine and the problem went away, and the engine did not appear to be damaged. It probably had less than a dozen runs on it before he stopped to figure out what was going wrong.
Dave