First of all I need to point out that I am very new at cycling. My issue that I have been having is that my chain keeps slipping whenever I have to push down harder than usually such as at a stop sign or hill. The bike has a new chain and a new cassette as well. It feels like it slips in the front. I almost wrecked the other day because of this.: Also whenever I change gears the chain goes haywire for a second. I had it professionally tuned up but it still does it. Can anyone give me any advice?

Does the chain have a quick link or was it joined with a chain tool?
If it has a quick link, remove the chain and reinstall it with the link facing the other way.
Also make sure all the links move/flex freely and are not binding.

Next suspect would be the freewhell/ freehub slipping.

Thank you for the advice. I will try reenstalling it tomorrow morning. Hopefully it helps.

How old is the bicycle? Are there a lot of miles and or abuse on the front chainrings? Did it skip at all before having the chain and cassette replaced? I'm thinking that the chainrings are worn out and the new chain does not engage the teeth properly and causes slipping under pressure. Try shifting in the front to a chainring you don't often use (I'm assuming the crank in the front isn't a single) and carefully try to cause the chain to slip. If it doesn't slip in that gear, it is a good indicator that the other gear is worn out and needs to be replaced. It is also possible that the chain may have a stiff link that is causing the problem too. I think the easiest way to determine this is to run the chain slowly and watch as it passes through the rear derailleur. Since the chain has to make a sharp back and forth motion there, it should be easy to spot a stiff link. The shop you had tune up your bike should have caught this issue (granted they test rode it afterwards) and made you aware of it before they returned it to you.

The bike only has about 100 miles on it. The only reason the cassette and chain were replace was because I bought a new set of whells for it and the bike is a 7 speed so I had to get a new cassette for the new wheels. Then I put a chain with a quick link on. It's not a real expensive bike so I'm sure the crank parts aren't of the highest quality. Thanks for the advice.

If it's new and skipping in the front, I would guess a stiff link. Why did they replace the chain if it is brand new?

I put a chain with a quick link on it.

So I swapped the chain around and looked for any stiff links. Today I took it out for a ride hoping it would be better. I actually think it's worse. I think it's slipping on one of the teeth some where on the crankset. It's a 7 speed so I am having a slight difficulty finding a new one for fairly cheap. Does anyone have any suggestions?