When I am pedaling my bike especially in the higher gears (5-8) the bike jolts like it is going to shift but doesn't. It locks up sometimes and my knees go into the handle bar. I think there is a problem with my back shifter.
Help?!

It's highly unlikely to be shifter problem. You should check the condition of your chain, derailler and freewheel.

This "trying to shift" problem is usually an indication that the cable needs to be adjusted or excess friction in the cable housing. You can try adjusting the cable at the barrel adjuster. If that doesn't work you probably need new shift cables and cable housings. The short housing found just before the rear derailleur often becomes dirty from picking up dust and dirty spray off the road.
www.parktool.com has good instructions for cable and derailleur adjustments.

if the shifter hesitates to shift up it's likely that there is to little cable tension.

Don't mean to sound nit picky but it's just the opposite for standard rear derailleurs. An upshift is to a smaller cog.

i believe any UPshift would be to a smaller cog, not which way the spring pulls the RD. Increasing tension on a low=
normal (conventional) RD moves from smaller toward larger cogs, while increasing tension on a top-normal RD moves
from larger cog toward the smaller cog. I hope I have that right.

Everything here is correct except you're mixing up top normal and low normal designations.

A conventional RD, in which increased tension moves the chain to a larger cog, is referred to as top-normal as the RD naturally (in the absence of cable tension) sits at the highest/top-most gear (albeit smallest cog). A low-normal RD naturally sits at the lowest gear (albeit largest cog).

Burf, if you're still here hit up the "repair help" section at the www.parktool.com link given here and up in that second post. Great help to someone that obviously needs some basic guidance. And if it's not just a minor adjustment then figure that the cable and housing to the rear derrailleur is sticky. You can try pulling the cable out and cleaning it as well as blowing some brake cleaner through the cable housings to clean them out. If your shifter cable is all kinked or frayed then invest in a new cable. If it's still not working after the adjustments and cable and housing cleaning then something is bent or worn out past reasonable wear amount and your best bet is to then take it into a bike shop.