No, unlikely to be U177, U891, etc, in that case. Sounds as though the system is waiting an unusually long time for something if there's valid CPU activity.
If you reduce the system to bare basic, e.g. A3630/A3640 CPU board (with J100/J104 set correctly) and nothing connected except the power supply and monitor; do you get the 'insert disk' screen after about 30 seconds, or does it take 8 minutes?
If the 'insert disk' screen appears after the typical 30 seconds or so, then probably there's an issue with drives as others have already mentioned; you should be able to narrow things down from there.
If the 'insert disk' screen takes 8 minutes to appear with no floppy or hard drives connected, then there's likely some other problem with the main board at a hardware level. It gets fairly difficult to diagnose yourself from that point. I use special run-time monitoring equipment for quickly diagnosing that sort of problem to see what's happening with the ROM software, so you can contact me separately if it's looking to be a main board hardware issue that you need repaired.