I have a 1998 <NOBR style="COLOR: darkgreen; FONT-SIZE: 100%; FONT-WEIGHT: normal" id=itxt_nobr_0_0>chevy</NOBR> ck1500 with a 4.3l vortek and 4l60e trans. Driving to work one morning the trans acted like it popped into neutral, when I got below 50mph it went into third and stayed there. I could manually get second and reverse but no first or fourth. Also the check engine light went on. If I shut the truck off then restart it drives normal for a few hundred miles then it happens again. I checked the code and it came up "shift solenoid B electrical circuit". I replaced both A and B solenoids and it still does it every couple of hundred miles. took it to a trans shop and they think it might be the computer but not sure.

 

That is in the year when GM trucks had problems with the ignition switch, it would drop the trans circuit off therefore killing power to the solenoids. The switch is relatively inexpensive and maybe worth trying.