MX5 engine problem

My MX5 NB of 1998, 93000 km, which I drive without problems for over a year now suddenly shows a weird problem. The coolant level in the reservoir is at minimum, filling it is no solution, after 2 days, minimum level again. Level is actually about the level of the joint clilinderhead-motor block. No leaks in the garage or at parking lots, no visible leaks either.
In the radiator, with the radiator cap unplugged and the motor running idle, there is some coolant movement which does not seem to be normal. Motor is shaking too much when running idle and produces not enough power when driving. Starting with cold engine takes a while, which was not the case until a few weeks ago. A pressure test on the cilinders was done showing cil. 1 = 3.5kg, cil 2 = 5.8 kg, cil 3 = 6.4 kg en cil 4 = 7 kg. Not enough pressure ... However, in hot condition cil 1 = 6 kg.
First idea was a leak in the joint between cilinderhead and motornblock. Cilinderhead was dismounted, no visible damage to joint or cilinder head no signs of coolant leak. Valves of the first cilinder had another color though, probably due to coolant in the cilinder? Cilinder itself was OK. Cilinderhead has been tested for leaks and is OK! The cilinderhead was 0.1 mm uneven, but I was told this is not a problem. Mazda mechanics also tell me the MX5 never ever has a problem with joint or cilinder head.
Has someone a similar experience and what can be the problem?
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Most obvious first guess is a shot head gasket, maybe a cracked or corroded head. Whip it off & take a look.
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Mal Osborne

Not really-US 99 models were 98 models in other countries. The clue is 93000 km and other metric measurements.
If the head was removed and replaced, plug wires are suspect, as are things that were disturbed, such as the cam sensor.
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