Everything you describe sounds like voltage problems. Your Mac is out of whack. Get yourself a multimeter and go to work, starting with Dr. Lee's excellent notes. Then get yourself a copy of Larry Pina's must have book for the vinate Mac enthusiast: Macintosh Repair & Upgrade Secrets, which will give you the alignment guides, and in some complete books, the software to make adjustments easier. There is also a step by step tutorial for adding an exhaust fan in the top rear chamfered vent opposite the analogue board –- the most popular location without impeding airflow. However, if you install a fan, you will want to construct a plastic dust shield to surround the 400K disk drive so as not to draw dust and debris through it further gunking-up the temperamental drive (as well as encouraging air-flow through proper channels).

Dr. Lee's guide will get your Mac back into top technical shape and help you determine if additional problems persist. More than likely you will need to start replacing aging components, as detailed in both sources.