Somewhat common problem with lower quality gas. Have you tried premium? I know that your manual doesn't specifically say you have to, but may be worth a try. Your question in regards to the ECU from a later version 850 fitting your car has been asked before, but unfortunately the answer is NO. You might fit a '93 or even an early '95 but no others. The ECU and engine management as well as OBD are completely different. Actually, 4 systems are used 850's...Fenix 5.2 (for older 10 Valve Turbo's...we don't see much in this couuntry), Motronic 4.3 and Motronic 4.4 and the LH3.2- Jetronic system. The Motronic systems all use a single ECU to control the fuel and ignition functions. The LH3.2- Jetronic system (in your 850 (unless yours is a 10 valve- Fenix), the '93's and some early '95's) uses 2 ECU's- one for the fuel and another for the EZ-129 K ignition system. Your system uses 2 black diagnostic boxes under the hood to deal with these 2 systems and their fault codes. The Motronic systems use the mandated OBD-II system to read codes, etc. through a connection within the cabin. Hope this information helps.
--
1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon