Possibly a coolant leak that seals up when it cools down? Metal expands when heated and crack opens up, then contracts when cools, seals the leak, and collapses the hose under vacuum? I'd bet its running hot because the leak is venting the pressure when hot. Head cracked?

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Matt
2003 Jetta Wagon TDI 145,000MI
1999.5 Jetta TDI 287,000MI
2001 Suzuki SV650S 26,000MI
2008 Yamaha Vino 125 11,000MI
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