Need help with my 2006 Saab 9-7x

Your ecu is either knackered(rare) or more likely, an earth cable is failing or loose-check all earth wires.

Checksum errors are usually from corrupted flash/eeprom area's in the module in questions. Sometimes you can re-flash them to fix the problem. Any shop with a j2534 tool can re-flash it using GM SPS ($120-$150). The engine computer (I am assuming your issue is with) is not that expensive used ($50-$100) so you may be better off to just have them flash a replacement instead in case the original is faulty and not just corrupted. The 9-7 saabs are not quite the pain to work on as their older models but you may still run into some resistance from shops when you mention its a Saab.