starting

for the last couple of months when I start my 92 jimmy 4.3l vortec I would turn the key the rpm's would go up then drop down almost to a stall but the rpm's would go back to normal and everything would run fine the last week when I start her up the rpm's go up and come down like it did before but it does stall but if a turn the key with the pedal on the floor then take my foot off the pedal it wont stall and this is only at start up any thought on what this could be would be greatly appreciated thank you

In my opinion, it is most likely the throttle positioning sensor. It is located on the side of the throttle body. Thats what I would check.