My Volvo won't start....I've checked the battery, it's fine. I've had issues past

If "jiggling the gear" is what's helped in the past, I would suspect the starter inhibitor / neutral safety switch or related wiring which appears to be part of the shift lever itself. Reviewing the workshop manual in brief - this does not look like an easy job to perform. The vehicle must be lifted and lowered to access various components, then the shift lever trim accessed, the shift lever removed and then disassembled for inspection. I would very much recommend a professional test and repair. Good luck!