I built a Hicapa 5.1 last night out of spare parts and I was getting the same issue. First mag or two worked so-so...then either excessive gas vent with each shot or else big gas dump on the first shot.
Now, I've never used anything but AI 1.5wt silicone oil in the mags to keep them lubed...and I probably use way too little...and have probably neglected them more than once. But I shoot pretty regularly, so all in all they stay ok.
I've definitely been guilty of using too much silicone oil to lube the blowback unit and trigger/hammer assembly. But like MadMax said, silicone oil "dries" up and I just wipe any excess off.
In the pistol I built last night, the hammer assembly was pretty much bone dry. There was some grease crud that had dried up, a little graphite powder from when I tried that out once, but otherwise no real lubrication. Still works ok. I probably should lubricate the bearing surfaces...
The venting was due to a metal upgrade bulb and spring that I had laying around and I had put into the nozzle assembly to replace the stock one. On the HiCapa nozzle, there's a little clip that holds the bulb and spring into the nozzle (not sure on your model). At first I thought that I had screwed in the retaining screw too far and it was stopping/clipping the bulb inside. But that wasn't the case. It's supposed to make each shot harder/faster.
I ended up swapping back in the stock bulb/spring and the pistol shoots "normal".
Anyway, there's a bunch of things that can go wrong on a GBB and cause it to vent or misbehave. Mag to chamber fitment, broken/cracked nozzle, stuck bulb/reed, sticking main valve, etc.... Unless you're pretty sharp and feel comfortable interpreting parts diagrams, translating that into what does what and then taking your pistol apart...take it to someone who can do that for you. (not me...I barely understand what I'm doing...
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