history: bought car used, radio did not function, factory amp was believed to be the issue and removed by previous owner

Recently searched and found part # for replacement amp, bought used one from parts reseller on eBay. Plugged in with no issue, speakers now get power, BUT cut off after 30 seconds. If power cycled off and on, speakers again work, but cut off 30 seconds again.

Any ideas at all as to where to look? Could it be a fault in the head unit itself? I previously had an E36 that had similar issues, replaced head unit and it went away.

Any helps appreciated, Thanks

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2000 x5 4.4 (production date 11/99)