with a scan tool you have no miss fire history codes ? however you say you feel the vehicle missing when driving. could be some electrical problem could even be the ignition switch which is a somewhat common problem in these at this age and a few years ago. some have had ignition switch power problems to the tranny as well. replacing the cap rotor appears to fix it for a while could mean the cam gear is worn this can happen.most of these engines 175K miles the distributor needs replacing. GM made the distributor gear of a material that reduces the wear of the cam gear . if a replacement distributor has a harder material over time it may have worn the cam. this would throw off the timing . with a scan tool it is possible to observe the timing when driving see if its erratic. measure the fuel pressures/test procedure . injector leak down will show as hard warm start . ECT can cause poor cold starts . if ECT is OEM factory it should be replaced too old. if it is miss firing you will get CEL flashes . possible the PCM is crapping out .