Rooz, you have already heard of many of the possible problems, so here is a way to pinpoint where the issue lies. You do need some special tools, but if you have mechanic friends it should be hard to get your hands on them.
1) Compression test. You need the proper compression tester guage and hose hookups. Remove all spark plugs. Jam throttle WIDE OPEN. Hook up tester and crank engine 5-6 compression strokes for the cylinder being tested. Repeat test moving down through all the cylinders. If you get a bad reading on any cylinder perform a WET TEST. Wet test consists of squirting oil into the cylinder and performing test again. If the bad cylinder pressure jumps up then that cylinder has bad rings. If the cylinder stays low then you need to move to the LEAK DOWN TEST.
2) Leak down test. So you completed the compression test and you have a low cylinder. You performed the wet test and the results were the same. This tells you that the cylinder in question does not have bad rings, and that compressed air is leaking out somewhere else.
This test is a little more challenging so take your time. Set the engine to TDC compression for the cylinder you are testing. Hook up the leak down tester and make sure that the engine does not turn over. If the crank rotates then you were not exactly on TDC and you need to find TDC again and start over. The leak down tester will tell you what percentage you are leaking. 20% is normal for an engine. With the leakdown tester properly hooked up, and the guage indicating severe leakage you can now diagnose where the problem lies. First listen out the exhaust. If you hear air then your exhaust valve is leaking (shouldn't be your problem). If you hear air coming out the intake manifold then your intake valve is leaking (also shouldn't be your problem). Next, take off the radiator cap and check for air bubbles. Be careful removing the cap. If you do indeed have a bad head leak, the coolant might shoot out all over you (I have seen this happen many times).
These two tests will tell you weather your problem is mechanical or electrical. If it is a mechanical problem then you will know exactly where. So, to recap first do the compression test. If everything is equal and high compression occurs, then you have an electrical problem. Do not continue with the leak down test. But if you have bad compression in at least one cylinder, and the wet test does not change your results (meaning that your rings are not the culprit) then you need to continue with the leak down test.
Sorry for the long post. Hopefully you can find some equipment to complete these tests. Let us know of your results. Good luck.