That is the best advice especially given that you have quite a few expansions.
Also check your 512K expansion for any leaking battery and corrosion around the battery which is one of the major killers of most expansion cards and Amiga's which contain an RTC.
You said it is an A500 right? If it is an A500 there will not be any batteries attached to the actuall motherboard but if it is a 500 Plus model you will need to open it up right away and check the motherboard.
You might unhook all the extras, and then try starting with a known working autobooting game disk, then add things in each time you power down. A beefier than normal power supply would be good luck too! Did you just buy this, or unbox it from ages past 
