There is a useful INIT that should sort you out with that problem and more:
PRAM Auto-Restore 1.0 - PRAM Auto-Restore is a Control Panel. To install it, drop it onto your System Folder. I've tested PRAM Auto-Restore with System 7.5.1 and 7.1, but it should also run with System 6.x. PRAM Auto-Restore resets your PRAM and Extended PRAM at startup and/or shutdown to a default that is stored in the Control Panel. It uses very few memory; when you let it reset your (x)PRAM at startup, it does not remain in the memory. When you let it reset your (x)PRAM at shutdown/restart, it installs a small (528 bytes) shutdown procedure (with the (x)PRAM defaults). You can set these defaults in the Control Panel to the actual settings in the PRAM by clicking only one button.
You can find a copy in lots of the 1990s archives (SUMAC, UMich etc). If you are stuck, give me a shout.