Heating Tempature

[jchase1126]

I have a 91 Mazda B2600i pickup. The temperture drops after driving the veh 15-20 mins. While sitting idle the temp rises but just as soon as I drive again the temps study drops and I'm getting little to no heat with the heater on. I noticed a drip coming out of the weep hole on the waterpump so I replaced it as well as replaced the thermostat but that has not resolved the lost of temp. Any suggestions?

[Tyranitar]

Your heat core is probably plugged. What kind of thermostat did you use, and did you replace the radiator cap?