Did I buy the right shifter?
 

Recently my wife bought me a Diamondback-
$8 from the goodwill- Diamondback Outlook!

I realized the Outlook was the lower model to begin with, but the price was definitely right in this situation. Especially since the tires are good and it worked just fine.

It worked fine anyway till the rear shifter broke.
The SRAM grip shifter is labelled MRX 200-10

I went to the local LBS who sold me a SRAM 3.0 Comp and claimed it was compatible.

I put on my bike and took it for a test ride.

It seems to me either I need to perform some more fine-tuning, or this shifter is not compatible with the cassette I have. Because it shifts fine on the first two or three smallest cogs, but is not reliable on the bigger ones- it gets to the biggest when the shifter is at detent #3 and it's hard to get to the next to biggest cog reliably.

Do I need to get my allen keys back out and pay more attention to the job I did, or do I have the wrong shifter?

Of course, though it seemed to work fine, I don't know if the shifter I had before was actually truly and officially correct for the possibly unknown cassette.

Looks like the bike shop sold you the wrong type. This ad says it's only compatible with SRAM's 1:1 ratio "ESP" rear derailleurs: http://www.cambriabike.com/shopexd.asp?ID=97507

The MRX 200-10 seem to have been paired with Shimano rear derailleurs, which means they must use the 2:1 cable pull ratio.

You want the plain 2:1 ratio shifter (MRX Comp or similar). Here the set of both left and right is selling for $15 for 7-speed: http://www.amazon.com/SRAM-Comp-Bicy.../dp/B001A0AIIS

Here's the rear shifter alone: http://www.amazon.com/Sram-Comp-Twis.../dp/B00280GG3U

Very nice find at the Goodwill!

SRAM 3.0 Comp shifters are intended for 7 speed freewheels/cassettes. Count your rear cogs. If you have 6 or 8, you have the wrong shifter.

If you do have 7 rear cogs, you can begin troubleshooting, or more likely completely redoing, your installation. Visit www.parktool.com for info on setting up shift levers. It sounds like you have multiple problems with your installation. Did you make sure the shifter was set for the smallest (highest number) cog when you connected the cable to the derailler? Have you made any effort to adjust the cable tension for proper shifting? Did you verify the high/low limit screws? Park Tool will cover all of this in detail but hopefully this have given you some things to think about.

[edit] Mondoman makes a great point about the MRX shifters that I missed. [edit]

OK, I'm not crazy and I do know how adjust a derailleur.

I adjusted the H and L stops, cable tension, multiple test rides and tweaks.
Then I did some googling and confirmed what Mondoman had already told me- thanks, by the way ;)
Then I took my new Comp 3.0 back to the LBS.

After a bit of "Well, this is really the only SRAM blah blah..."
I explained that it just seemed the pull ratio was wrong- there's a 2:1 and a 1:1 and I think I need 2:1 and this is 1:1. LBS guy did a bit more looking at his screen and typing and determined the 3.0 is indeed 1:1 and works only with SRAM cassettes.

In the end he was fairly cool. He ordered the right part which will arrive Thursday.

So I guess it's all OK. I paid a bit more bicycle tax than I meant to, but it's going to support a local business so I guess I'll survive.

If you have Shimano derailleurs you could also go with any Shimano shifter with the right number of speeds.