On my 91 f150 5.0 V8, the engine hesitates when accelerating and now has begun bucking now that the weather has warmed up. This is not a problem in cold weather (winter in NC) but was a problem last summer too. The temp gauge is with in normal range but higher than in winter. Addition info possibly related is as follows. The engine burns oil as well as blows oil into the air filter chamber clogging the air filter over time. The oil pressure gauge is in normal range when driving (foot on the gas) but drops off the low pressure side when stopped after initial ~5 miles of driving (it returns to normal range as soon as accelerator is pressed.
Any ideas on the cause?

 

A couple of thoughts..

Have you pulled any codes? Has the CEL been on @ any time? If you don't have a code reader, there is an easy way to pull the codes from the PCM for free using a short piece of jumper wire. It can also be done with a Analog Voltmeter. Have a read from the "STICKY" @ the beginning of the forum and you can also chek this site as it is loaded with very useful info.
Ford Fuel Injection

From your description about the oil blow-by in the breather element, it sure sounds like worn rings. Could also be that your PCV valve is plugged/gunked up/worn out. You should inspect it or at the very least just get a new MOTORCRAFT PCV valve if it hasn't been serviced in years. Do not purchase a cheaper version of the PCV valve as it may allow for excessive amount of vaccum leak. What weight oil are you running? You could benefit from some Rotella 15w-40 or similar 20w-50.

Last but not least...If you are basing your oil pressure result from the stock gauge, you are doing yourself a dis-service. As one member stated.."It's nothing more than a Glorified Idiot Light.." and it is. The sender registers @ 7psi, and that can correlate anywhere between "NORMAL" on the gauge. It uses a resistor in the dash cluster. If you really want to know the real oil pressure, buy a $10 gauge and mount it in the engine bay. If you want it in the cab, I suggets a digital gauge or to use a small copper line to feed from the block to the gauge, IN case the nylon line decides to burst(from heat/cold/stress). Oil blowing over everything in the cab isn't a pretty picture.

Get back with your results and we'll do what we can for ya!