Almost every 103 cylinder head I ever rebuilt or re-surfaced had the gasket blown on #6, exhaust side. If you get a minute pull the spark plugs and compare #6 to the rest. Even just a few drops of coolant will discolor the plug, even if the car is not noticeably using coolant. When you determine it is the gasket, bring it down, I'll fix you up for a fraction of what a shop will charge. 
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1987 300TD Turbo 5-spd
1991 350 SD Turbo (Frankenstein 603.970 crank, rods and head on a .50 bored 603.963 block  )
1982 300D euro 617.951, 4-spd
2003 E500
2000 GMC C2500 6.5 TD (Double ott, ex DOT truck)
1980 Scout Traveller SD33T (Resting!)
1997 GMC K2500 Burban 6.5 TD (mid engine rebuild)
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