I noticed a lot of posts having the same problems with their engine(3.3L)hesitating to the point of almost (or actually) dying at speed as well as idle. My problem was solved from a computer diagnosis giving me a code telling me that my camshaft position sensor was at fault. The CPS is located under the distributor cap and couldn't be replaced alone, so a complete distributor needs replacing. Local parts stores carry the unit to the tune of $320.00. Ebay saves you money from a place called Distributor King (?).
A few problems with the distributor were present. An LED sensor on the CPS will give an eratic signal ifthe unit is dirty. Also, a bearing showed much wear on the distributor shaft which I was told at part store is a cause of the same problem. You can attempt to clean the foil LED disk under the cap, and possibly get positive results. If the bearing shows excess wear, which will be obvious by the whirring sound it emmits, as well as the play in the shaft, you must replace the entire distributor. Again, this was the solution for me, and your problems can be fuel related, as well as any of the other dozen or so sensors that control your drivability. Don't have a mechanic use the process of eliminition to narrow down the problem. Get a computer diagnosis to spit out trouble codes and go from there.