Deutz aircooled overheating

Hi all,
My main genset, a 2cyl aircooled Deutz 410, 12hp @1500 rpm, 6KVA alternator was overheating and cutting out when running at 75% load for
30 minutes. So I checked and the cylinder fins were completely packed/caked with an oil/dirt/sawdust goo...The temp gauge read ~ 70 C.
I cleaned them out, cleaned out the oil cooler and life was good for a couple of months, runs at 75 % load for a couple of hours...(long hot water baths...)
Now its doing it again, but the temp gauge is only reading ~40, (as it always had..)..
The oil in the crakcase is "very hot" to the touch though.. The fins are clean Have not checked the oil cooler tube yet The belt driving the fan is okay.. Oil pressure seems normal (~90 psi cold, 35 psi hot).. Done ~ 6000 hrs since last top end rebuild (valves/rings)
Any ideas, outside of bypassing the temp sender:-(
regards nicolas boretos
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