2005 Blazer S10 4.3 Auto. 2WD 96,675 miles, on 5-27-18 while towing my boat home I stop at gas station approx. 55 miles a/c on outside temp about 90 degrees 18 foot bass boat in tow, I went in to get a drink and left blazer running, I returned approx. 7 min. later to find that my temp gauge was about 3 notches past the 210 degree mark for first time ever. I am the only owner. and have been **** on all preventive and schedule maintenance . When I left station clutch fan was roaring it took temp down to 210 degree's and then started to quiet down, so fan appears to be working. The blazer never did go any higher in temp since then and did not overheat. Now this is what I have done since 5/27 (1) I super flushed radiator for 150 miles also letting it run for 1 hour at fast idle. Flush system out with fresh water 3 times @ normal temp to ensure i got any crud out.(2) I changed the Thermostat used new GM. (3) Replaced Temp. Sensor on drivers side of block new GM.(4) Replaced Radiator Cap new GM.(5) Put new Regular anti freeze. Check system all week did loose any antifreeze, when I removed Rad. Cap to check always full to top. This week it has gone past 210 degrees 1 line at highway speed with out pulling boat and it has never done that before and again the clutch fan kick in to bring down temp. There is no steam or heavy white vapor coming from exhaust tail pipe. Has anyone experienced this problem on a 4.3 Blazer ? Thanks in Advanced Ken