If you do not install the RAM in the correct pairs, the system will think you have more RAM than you actually do. In this case, you have two 16 MB SIMMs and two 4 meg SIMMs, which would normally give you 48 MB of RAM. However, if you insert these as 16, 4, and 16, 4, the system will think it has 32 + 32 + 8, or 72 MB of RAM total. Improper installation of the RAM can cause various system errors such as you are experiencing.
The Power Macintosh memory sizing algorithm looks at the highest capacity memory the SIMM bank can handle. It then assumes that both SIMMs are the same size. If one is not correctly (equally) sized, then the computer will present you with a system error as soon as your system and application memory usage goes beyond the 8 MB memory that is on the logic board.