I'm at my wits end and about to give up on a second hand dual suspension I bought a few months ago. The bikes history is it fell off he back of a bike rack but not at high speed. The gears gave trouble after that, even snapped a few chains. I decided to buy it to fix up and use myself. I took the drivetrain off my hardtail and basically swopped the parts. The bike actually worked fine for a few months until it needed a service. I decided to take it in, I was told the chain needed replacing, the cassette needed replacing, the gear cable housing and the BB set as well. The parts were all supposedly replaced, even though I later realized they hadn't all been but lets leave that rant for now. The gears ended up being a disaster, the bike wouldn't even change onto big chain ring. I took it back and when I tested it the chain kept being thrown off the big chainring this time. I decided to take it elsewhere as I didn't even have the patience to go back there again. I took it to another shop that were at least a lot more helpful and made an effort to try fix the bike properly. They said they thought the problem may have been solved but they didn't even want to charge me until I was happy, they said the problem could have been the derailleur hangar was bent which made sense actually considering it's history. I paid them anyway because I appreciated their honesty. The gears were better but slipping on big chainring, I took it back and asked them to give it a proper major service which I was happy to pay for even though it had recently been 'serviced' . They changed the rear derailleur wheels and the gear cable housing as that hadn't been changed after all. The gears are however still slipping in big chainring, specifically under load. I don't really know what to do now but I can't keep throwing money and replacing parts as the whole process of elimination is getting a bit expensive especially considering its an old bike. Is there anyone who thinks they can sort out the problem, I'm at a stage now that I might just sell the bike as is, and disclose the trouble upfront and pick my battles rather? I would actually upgrade the whole drivertrain to a 1*11 or 12 but I definitely don't want to go to that expense if the problem persists. Appreciate any input you may have.