I have a Trek Navigator (comfort bike) that had grip shifters. I ran into the same problem, inadvertent shifting, usually at the worst possible time, climbing a steep hill and standing on the pedals. I recently replaced the rear wheel, the cones in the hub were going bad after 9k+ miles (new wheel has cartridge style bearings). The original wheel had a 7-speed freewheel. The replacement wheel has a freehub that I put a 8-speed cassette on. I replaced the grip shifters with SRAM X-4 trigger shifters from REI. I also put on lock-on grips that are rock solid and do not move. Big improvement. The only thing I didn't anticipate is the chain line changed a little, the new set of rear cogs is slightly more inboard and shifting to the 3 innermost cogs (lowest gears) with the biggest chainring doesn't work well. However, the smallest chainring now works better with all 8 cogs on the cassette, which works out better when I go up unpaved trails. The only caution I have is make sure the trigger shifter you acquire is compatible with your rear derailleur. The SRAM rear derailleur I have requires a matching SRAM type of shifter because the actuation ratio (amount of cable pull), is different from Shimano derailleurs.