1989 Jeep Cherokee hesitation and start up

Hello, I have a 1989 jeep cherokee, with a couple issues. I have had it for about 2 years and I have experienced engine hesitation and lagging on start up. Hesitation sometimes occurs while driving. I would drive for awhile and my engine would start hesitating without a known cause. I took it to a shop and the mechanic replaced the dying fuel pump. My car ran very nicely for about 6 months and started showing hints of hesitation, then while driving on a rainy day, my car began hesitating a whole lot and it couldn't really get anywhere, so I had a tow truck come and take it back home. The next morning I tried driving it and ran just fine! I don't get it, so I took it to a shop and the mechanic couldn't notice anything! *Sigh* also, my engine lags while starting;i t's weird because it usually happens on warm and sunny days, around 70+ F degrees, I don't know if it's due to the crankshaft or starter, or something. So, I was wondering, what causes the random and mysterious hesitations and rough start ups of my engine and what can be done to fix these problems? I highly appreciate any feedback, thanks.