either your shifter module (the whole shifter unit) has failed and requires replacement (most likely from spilled liquidds, as stated here). Or, you have a stop lamp switch that is reading an intermittently implausible value. Might as well start with the stop lamp switch, because it is cheap and about a 5 minute job to replace. if that doesn't fix it, put the old one back in and save the switch (trust me, you'll need it soon enough). Then, bite the bullet and pay for the new floor shift module. 99% of these problems are caused by one of these two things. I have seen a couple diaphram pressure senors that have been fries by brake fluid leakingin and shortin them out, but this was on 203/209 cars, and is not common even for them.