If the expansion valve clogs, you will see frost around the outside of the valve and the low side pressure goes very low (zero or below). Therefore, clogging valve is not your problem.
It looks like overcharging is not the cause either that narrows down to "expansion valve stuck open" or "air in the system".
Keep in mind that when the system is just full, you will still see bubbles (also, see Benzmac's post under the subject of "A/C readings") during:
(1) compressor engaging/disengaging;
(2) you are varying the engine rpm rapidly when the compressor is engaging.
You use R12 out of a brand new can (or tank), isn't it?