Ahh... short term variations. If the engine is running lean, such as it would with a fuel pump problem, or air intake leak, then the idle wont sit at a nice eg 1000rpms from one second to the next. It will tend to flutter and surge between eg 950-1050 (or even more variation), over a period of a few seconds. Note that variation of 10-20rpms over such a period should be considered normal.

But if I'm hearing you right, sometimes it's steady at 1500rpm, and other times it's steady at eg 1800rpm. Not that it's fluctuating between the 2 within a short time. That would seem to indicate that it's not a lean engine problem, but rather that it's something more intermittent. You've already indicated that you don't smell any gas, and since you've confirmed the presence of an ECU, we can reasonably eliminate 'stuck choke' as a cause - that should all be handled by the ECU.

Time to check your cooling system... Patrick had a very good point about the water pump that I had forgotten about - and has reminded me that exactly the same thing had happened to my old golf when it was about 12 years old - the water pump went, resulting in random overheats and water loss problems... and yes, an increased idle is a symptom. I don't know how I forgot that.