Sounds like an overheating issue. They probably have thermal cutout protection built in. Take the PSU out, take the lids off and blast 7 years of dust out with a compressed airline. Put it back together and try it. Nothing to lose...
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Current:
99 Subaru Forester S-Turbo Auto - interesting... but it's not a quattro
91 Reliant Scimitar SST 1400 - no.39 of 42... something for the weekend
Previous:
Two consecutive 1989 Audi 200 Quattro Turbo Avant, 1B
1990 Audi 100 SE Avant, NF
1986 Audi 100 CD Avant, KU
1987 VW Passat GL5E Estate, JS
"Honorary" triple member of the 200,000+ Club 
Audi 200 in the UK (legacy website)
Classic Audi member no: 258
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