Rear cassette issue's...??? Any advice?


Hi! I just finished my second build on this Cannondale Trail 6, hardtail. Its been up and running for a little over a week, but as you soon realize it's never a done deal! There's always going to be little issues that arise, components that need to be tweaked, played with or shaved down(still talking about a bike ;-)...
Anyway, gearing has never really been a strong point for me. The lower gears seem to be working fine but when I get to 6 especially, I start getting some crunching and grinding.
I'm sure I probably have overlooked something(which comes natural to me)... Yesterday, I took off the rear wheel and tightened up the cassette which seemed to be loose. It helped, but didn't solve the problem. Maybe I'm just expecting too much because it is running pretty good, but ppl inquiring about possibly buying it, or one like it, don't want to hear, "that happens sometimes"!...
I had similar issues with the first bike but eventually figured it out and it was just a matter of time. This bike has better components and is better quality, so I'm thinking it can be resolved.
It is the stock 8 gear cassette that I am using, but I switched out the shifter and am currently using a seven-gear Shimano twist grip bc that was all that was available at the current time. The LBS mainly deals in custom made the cruiser type bikes and almost frowns upon electric. So anything that I guess I usually purchase online or through all the resources but I'm wondering if anyone can recommend something that would fix the problem indefinitely? Thanks again! Jason