The gunk in the cooling system is not a good start. I'd flush the cooling system out and use demineralised water and see what happens. If you get anything contaminating the water and/or still have the same symptoms I would then get a shop to do a pressure test.
Another possibility is a slipping valve guide. I had this problem in my 4G64, and couldn't have possibly detected it without removing the head and inspecting it well.
I offer that you try the more pedestrian tests already suggested. BUT, if the problem persists, consider my experience:
The symptoms were somewhat random, too. The main thing is that oil is allowed to seep down into the cylinder because the valve seal pops off its seat. I have a drawing of this, let me see if I can find it and post it for you.
Yes I found it, and the post regarding the condition. http://www.mightyram50.net/vbulletin...ll=1#post26192
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