Fluctuating Temparature

[meded13]

I have a 1991 Mercury 200HP EFi. Ive noticed that as I reach WOT for about a minute, the engine temp returns back down to what appears as zero (first line) on temp gauge . Engine then runs as cold, really rough, starts to smoke more than usual and stahls as I bring into slow idle or idle. This is causing the engine to run rich, thus fowling plugs. Ive replaced both T-stats, poppit valve and temp sensor. Dont know where to go next.

Thought I should mention that it runs like a beast WOT. Don't think it could be good for engine to run cold.

Would greatly appreciate any assistance with this one. Thx