We have a 1999 Dodge Ram 3500 van 5.9 liter. All of a sudden during a rainstorm, it started having trouble keeping a cruise speed on the highway. We thought it was plugs and wires, but no. Since then the check engine light comes on, then goes off. Runs worse to the point we can't drive it at all.
Have checked for vacuum leaks, replaced the MAP sensor, the air idle control valve, and had it the computer checked for codes at an auto place. Code was for MAP sensor and something they can't read. Can't afford to take it to dealership to pay $100 just for a code read.
This is driving me nuts!!! Has had tune up, new speed sensor...One new O2 sensor...any thoughts?

 

Welcome to the world of Mopar engine management electronics. Try changing the 2 wire temperature sensor located on the intake manifold between the A/C compressor and the alternator.

Access to the sensor is achieved by removing the alternator and then loosening the A/C compressor by unbolting the mounting bracket. Leave the refrigerant lines connected; just move the compressor out of the way to the left side of the engine.

The sensor is located near the thermostat housing where the upper radiator hose connects to the intake manifold.

Hope this gets you going again -- good luck.

 

IMO, the bell ringing symptom in your OP is the "During a rainstorm"

The engine computer is located high up on the firewall, and depending on the year, has multiple multi pin electrical plugs running to it.

I'd disconnect the battery, locate the ECM/PCM, pull the connectors and inspect them closely for moisture, or corrosion.

Clean them with some electrical contact cleaner, slather them with some di electric grease and re install them. Sometimes bending the pins slightly ensures better contact.
Do the same to every connector that you can find under the hood. Look for wires, or connectors that could get wet when at speed.

Also, remove and clean the battery terminals, and battery to engine ground, battery to firewall ground, and engine to frame ground.

 

I agree, and a cracked cap can get moisture in it during the rain and cause these exact symptoms. If the cap has been changed then I'd look into the MAP sensor to see why you're getting a code for it.

 

Indeed a cracked ignition cap can cause all this headache too. When you replace it, get a copper/brass cap-n-rotor set. The aluminum types are junk.