TM G36 power loss problem

[Imperial Guard]

I'm a proud owner of a TM G36, but my gun has a problem that keeps me awake at night:
I got a PDI170% in the gun last year and only test fired it. It was shooting at around 430fps with hopup off.
For this summer, I tested it extensively and the fps varied from mid 380's to mid 430's, averaged to be roughly 390-400fps with hopup off.
It started a couple of weeks ago, just before I considered de-tuning the gun for safety and reliability reasons, the fps suddenly dropped from 390-400fps (at noon) to just 335-365fps (in the evening) before I got a chance to touch the gearbox.

Then I tried a new Prometheus MS120 and the gun chronoed at around 380-390fps, but the same spring could hit up to 420-430fps on my buddy's AK.
Thus, I'm suspecting a leak/mechanical problem somewhere. But upon careful inspection of the whole gun, I just couldn't find anything wrong with it. And I'm very sure that all the parts are installed and lubed properly. For reference, here's my G36's setup and my buddy's AK's setup:

TM G36 (only non-stock parts listed)
Star 6.05mm/495mm inner barrel
Systema Area 1000 piston and aluminum piston head
Systema V3 bearing spring guide
Prometheus MS120sp (used to be PDI 170%), will switch to MS100sp.
10.8v batt
Systema metal bushings and shims.

TM AK47
Hurricane FTK M120s

I guess my main concern is: would the combination of aftermarket cylinder, cylinder head, and loading nozzle make such a difference (50fps increase)?
If it could, then I have nothing much to worry about; if it couldn't, that means something is f'ed in my gun. Any feedback?

[Identity]

What your guess is could be right. I don't know much about gearboxes but, I think I know a few tips that could help you with your AEG. I have a G36C too. However I don't have a cronoy to test how fast it's shooting. However, what I have heared is that some aftermarket pistons and piston heads are not that great, and the stock piston head + pistons work better than some aftermarket ones. Maybe you should put more litium greese on the pistonhead and see if theres any differnce? Could it be your shims by any chance? Not being aligned well and putting strain on the gearbox? Im just guessing. Maybe you should have a pro to take a look at it or analyise it.

[Kerbosa]

I had a problem with my gun after upgrading it that the hopup didnt fit the inner barrel properly and was loosing seal around it, causeing my gun to shoot ~40fps less than it should. check your nozzle/cylinder/cylinder head/piston seal, by assembling it as it should go outside of the gun, pluging the nozzle with your finger and pushing the piston through. you should have a tight seal and not be able to push the piston all the way to the head. (you may be loosing seal arround the piston) if that is fine, then check the allignment with the hopup as well. I had read about a problem with G36's regarding the hopup not lining up proberly so if you search around you can probably find something to that regard.

[Imperial Guard]

Thanks for the reply.
Upon a closer look, I actually found that the loading nozzle was kind of out of alignment with the hopup/inner barrel assembly. The hopup assembly was fine and I even did the seal enhancement thing (specified in the upgrade/modification section).

On the other hand, the nozzle on the cylinder head was loose (I could turn it while it should be sealed dead on). I guess that's the major source of the suspected leaking problem. I think I'll try after market cylinder set and see how it goes. I'm really hoping it's not the inner barrel/hopup chamber alignment problem.

[Kerbosa]

the nozzle should turn on the cylinder head. not a wobble back and forth but twisting shouldnt be an issue. test the seal on your piston/cylinder. that will tell you if you need to change something before you go and buy the parts.

[Imperial Guard]

Hmmm, good call, I'll for sure try it out. Thanks.