I've been having this issue for a few months, and it's getting more and more prevalent. When I'm pedaling with force, particularly when standing, the chain slips, which is annoying, counterproductive, and dangerous. Lately, it will happen I almost every other pump of the pedals. This occurs no matter which gear the bike is in, but for the record I usually keep it at the highest gear and highest friction. I've only had it for about a year, and I got it brand new. I would really like to fix this issue, since it's my only transportation.
If you're racking up the kilometers / miles your chain could be wearing out. This is easy to check. The link spacing on bicycle chains is 1/2" – there are tools for this, but you can get a rough idea with just a ruler – measure out 10 or 12 inches along the top of the chain. As the chain wears it will get longer and you'll see the pins moving apart. A chain is worn out when you're approaching 0.75% of "stretch." Roughly speaking this is about 1/8" at 12 inches. If a careful measurement show you something close to that you've probably found the culprit.
Replace the chain for sure, a worn chain also damages the cassette and the chain rings. All of which are more expensive than a chain. The conservative thing to do would be to replace the cassette along with the chain (and it sounds like you want one that gives you a higher top gear). If money is an issue start with the chain. IF the problem persists, also do the cassette.
I recently had my chain 'skipping' on my mtb and it is quite annoying while riding. I found that I had a link that was seized. I was able to press out the pin and repress it back in and it seemed to have solved the problem. I only have about 500 miles on the chain but they are rough miles, lots of stonewalls and rock in this area of the Northeast. I believe that my chain came in direct contact with a rock.
You will need to change the cassette, chain and possibility the chain rings on your bike. change all of them at the same time to ensure they wear out at the same time and you don't have a worn chain wearing out a new cassette at a faster rate for example (or vice versa).
You can also check the freehub as it may be worn out. This is what the cassette spins on and it can wear out. To change this you need a large allen key (I forget what size) and you'll need to remove the cassette first.