My transmission seems to slam into 3rd gear sometimes and for awhile it started slipping into that gear so i put some Lucas transmission fix it in it and it stopped slipping but now still seems to shift hard and the other day i was getting on the highway and it was in first and i was just giving it normal acceleration (not taking it above 2500rpm) and it reved all the way to 3500 RPM and wouldn't shift and finally shifted after a it got above that... i've had it happen twice before this but it never went above 3000 RPM. what can cause the trans to do this? and what can be done to prevent it from going out? thanks for any help and advice!

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1994 Ford Ranger 4.0L 5 speed 4wd
2009 Ford f150 4x4 v8
traded 2009 Mercury Mariner 2.5L I4 FWD
Sold:1996 Ford F150 4x4 300 6cyl 5 speed
Traded: 2003 Ford Ranger XLT Auto 3.0L V6 Flex fuel Extended Cab 2WD