I have a 3000 desktop behind me that does the same. Thing is I leave it running while I have breakfast and when I come back and reboot after about twenty minutes she generally boots fine. I kinda think its probably one of those pesky capacitors or something. Just needs to warm up.
I also have a 4000d here that locks up with the solid yellow. Both these machines are here cus I have moved them out of the colder workshop for the winter. The 4000 has a problem with the hard drive and even after formatting and checking thoroughly I couldn't find any errors. Turned out it is a problem with AsimCDFS which calls the CD-ROM and when I watch the light it doesn't go out at the critical moment and the drive then locks in solid. In the end I just removed the AsimCDFS from the start up. The light still locks solid but the computer boots and eventually the checksum cures itself.
I say all this cus when I have problems with any of my many Amigas the first thing I do is not mess. I just reflect on the last time it was working and therefore it must be something that has changed that caused the problem. If its booting off the floppy and you are able to check the early boot screen and the keyboard flickers and you can get a caps light after first boot I would tend to think its a drive issue. And if you haven't messed with anything I would guess its a problem with the drive. And with the solid I would just leave the machine all day or boot off the floppy and see what happens when the drive is checked thro tools or something. If the computer wont see the drive or it isn't in the early boot then you have to track the problem from a change in circumstances from when it was working and work back from the problem. By fiddling with many other things, particularly on old kit you may just make more problems and create a bigger issue trying to solve matters.
The bench behind me here has three computers with boot issues and I generally just sit and listen and wait and observe and coax them back. I have a 2000 that was flickering, stalling and refusing to boot and I stood it on its end and then gently tapped the drive and it kicked back into gear.
Its hard resisting messing but in truth if it was working and then it stops, its going to be a failure of probably one thing. And first step is just getting the computer running without really messing with anything or swapping anything out. ie unhitch cards, drives but only by disconnecting cables etc. I have a 1200 tower that developed an issue and only on close inspection did I discover that the tiny plastic back cover to the ribbon off the hard drive had worked free and the pins had come adrift. That gave the same issue.
Anyway have fun. I just wanted to say that with old kit it can get even worse when you start moving stuff about. You just don't know what you are likely to have problems with next.
No help as ever but I feel your pain. I get all mine working eventually, but normally after a good time of thought and observation of the problems. Plus a lot of research before I even start mucking around with the kit. In the end my guess is its something very simple. And it's in plain sight.. you just can't see it yet.